Negative reinforcement:
First Party - As DLC, they may not have a good chance of getting in since there has been a large focus on Third Party characters.
Assist Trophy - This is an automatic DISconfirm for a character. If a character does get promoted to a character then I'll be very surprised
Spirits - This is more of a soft disconfirm as spirits do possess a lot of importance in the game such as powering your character up, the main point of World of Light and being given away as prizes for online events. They may appear if they get lucky but i wouldn't count on them being frontrunners.
Indie character - While it's not an automatic disconfirmation, two of the most highly requested indie characters such as Shovel Knight and Sans were not made to be playable characters. However they are western characters and we have not seen a Japanese indie character yet so who knows.
Obscurity - While not the most definite factor of a character's chances, it can hamper their chances of making it if they are not well known enough.
Timing - This is a key factor of what allows a character to join. With Rex, he was made a Mii costume since his game was too recent to properly make a character.
Positive reinforcement:
Third Party - As DLC, They have the best chance of getting in since again there seems to be a larger focus of bringing Third Party characters into the game.
Unrepresented - If a series is currently unrepresented then they have a good chance of getting in. This synergises well with third party characters.
Protagonist - Most series are introduced into Smash Bros with the protagonist first. While Pikachu may be a grey area, you could argue that he is the protagonist of Pokemon Yellow. If the main protagonist is already represented then other characters may be available to join them.
Fan requests: If a character is highly requested then they will more likely be chosen.
Corporation influence: This is a sure fire way of having a character be chosen as it can show there is a mutual interest between Sakurai and the company he is working with.
_____________________________________________________________________________________________________________________________________________________________________
Here are the ranks given so you can CTRL+F to find which character is given this rank
Very Likely
(Put space here) Likely
Somewhat Likely
Balanced possibility
Somewhat Unlikely
(Put space here) Unlikely
Very Unlikely
Borderline Impossible
(Put Space here) Impossible
_____________________________________________________________________________________________________________________________________________________________________
Dixie Kong (Donkey Kong) = Very Unlikely
Reason: Already a Spirit
Toadette (Super Mario) = Very Unlikely
Reason: Already a Spirit
Midna (Legend of Zelda) = Extremely Unlikely
Reason: Already an Assist Trophy
Style Savvy character (Style Savvy) = Very Unlikely
Reason: Already a Spirit
Elma (Xenoblade) = Very Unlikely
Reason: Already a Spirit
Fiora (Xenoblade) = Very Unlikely
Reason: Already a Spirit
Byleth (Fire Emblem) = Balanced possibility
Reason: Currently unrepresented but a First Party character
Edelgard (Fire Emblem) = Very Unlikely
Reason: Not the protagonist.
Lyn (Fire Emblem) = Extremely Unlikely
Reason: Already an Assist Trophy
Caeda (Fire Emblem) = Very Unlikely
Reason: Already a Spirit
Lara Croft (Tomb Raider) = Somewhat Likely
Reason: Main Protagonist of game, unrepresented series, Third Party character, decent fan support, However must compete with more popular characters such as Sora and 2B.
2B (Nier Automata) = Very Likely
Reason: Unrepresented series, corporation influence, heavy fan support
Female alts for The Hero (Dragon Quest) = Impossible
Reason: Characters already represented in game as characters
Carmen Sandiego (Where in the world is Carmen Sandiego) = Borderline impossible
Reason: Very obscure, no fan support, must compete with a lot of other characters, even if she becomes popular it'll be way too late for her to join.
Jill Valentine (Resident Evil) = Likely
Reason: Unrepresented series, healthy fan support
Chun-Li (Street Fighter) = Very Unlikely
Reason: Already a Spirit
Shantae (Shantae) = Very Unlikely
Reason: Already a Spirit
Amaterasu (Okami) = Likely
Reason: Unrepresented series, healthy fan support
Morrigan (Darkstalkers) = Unlikely
Reason: While from an unrepresented series, it's more obscure than most fighting games.
NOTE: Cleavage could lead to censorship.
Nakoruru (Samurai Showdown) = Unlikely
Reason: Already shown in a character trailer but could still make it with enough fan support
Ribbon Girl (ARMS) = Very Unlikely
Reason: Already a Spirit
Fio or Eri (Metal Slug) = Somewhat likely
Reason: Unrepresented series but series not as popular
Impa (Legend of Zelda) = Very Unlikely
Reason: Already a Spirit
Jeanne (Bayonetta) = Very Unlikely
Reason: Already a Spirit
Terra (Final Fantasy) = Somewhat Unlikely
Reason: Although from an unrepresented game from the series, not a very popular pick.
Tifa (Final Fantasy) = Somewhat Likely
Reason: Main character already represented so there is room for her to join, however not the most popular pick.
Ultimecia (Final Fantady) = Very Unlikely
Reason: Although from an unrepresented game from the series, not a very popular pick.
Chie (Persona) = Very Unlikely
Reason: Not the main protagonist
Naoto (Persona) = Very Unlikely
Reason: Not the main protagonist
Susie (Kirby) = Very Unlikely
Reason: Already a Spirit
Tetra (Legend of Zelda) = Very Unlikely
Reason: Already a Spirit
Vivian (Super Mario) = Very Unlikely
Reason: Already a Spirit
Sheeda (Fire Emblem) = Very Unlikely
Reason: Although she has no spirit, very obscure character with no fan support
Celica (Fire Emblem) = Very Unlikely
Reason: Already a Spirit
ZX (Mega Man) = Very Unlikely
Reason: Already a Spirit
Tron Bonne (Mega Man) = Very Unlikely
Reason: Already a Spirit
Kumatora (Earthbound) = Very Unlikely
Reason: Already a Spirit
Makoto (Street Fighter) = Unlikely
Reason: Although no spirit, not a very popular pick for DLC
Ibuki (Street Fighter) = Very unlikely
Reason: Already a Spirit
R.Mika (Street Fighter) = Unlikely
Reason: Although no spirit, not a very popular pick for DLC
Juri (Street Fighter) = Very Unlikely
Reason: Already a Spirit
Menat (Street Fighter) = Unlikely
Reason: Although no spirit, not a very popular pick for DLC
Poison (Street Fighter) = Unlikely
Reason: Although no spirit, not a very popular pick for DLC
Sakura (Street Fighter) = Very Unlikely
Reason: Already a Spirit
Melia (Xenoblade) = Very Unlikely
Reason: Already a Spirit
Shanoa (Castlevania) = Very Unlikely
Reason: Already a Spirit
Sypha (Castlevania) = Unlikely
Reason: Although no spirit, not a very popular pick for DLC
Yoko (Castlevania) = Very unlikely
Reason: Already a Spirit
Charlotte (Castlevania) = Very Unlikely
Reason: Already a Spirit
Blaze (Sonic the Hedgehog) = Very Unlikely
Reason: Already a Spirit
Captain Syrup (Wario Land) = Very Unlikely
Reason: Already a Spirit
Ashley (Wario Ware) = Extremely Unlikely
Reason: Already an Assist Trophy
Mona (Wario Ware) = Very Unlikely
Reason: Already a Spirit
King (King of Fighters) = Very Unlikely
Reason: Protagonist (Kyo Kusangi) has not appeared yet
Leona (King of Fighters) = Very Unlikely
Reason: Protagonist (Kyo Kusangi) has not appeared yet
Shiki (Samurai Showdown) = Very Unlikely
Reason: Protagonist (Haohmaru) has not appeared yet
Blue Mary (Fatal Fury) = Somewhat Unlikely
Reason: Protagonist (Terry Bogard) has appeared in Smash Bros and is unrepresented in game. May appear as a Spirit though.
Mai (Fatal Fury) = Somewhat Unlikely
Reason: Protagonist (Terry Bogard) has appeared in Smash Bros and is unrepresented in game. May appear as a Spirit though.
Mature (King of Fighters) = Very Unlikely
Reason: Protagonist (Kyo Kusangi) has not appeared yet
Miriam (BloodStatined) = Unlikely
Reason: Indie character, genre already represented by Castlevania
May (Guilty Gear) = Very Unlikely
Reason: Protagonist (Sol Badguy) has not appeared yet
I-No (Guilty Gear) = Very Unlikely
Reason: Protagonist (Sol Badguy) has not appeared yet
ABA (Guilty Gear) = Very Unlikely
Reason: Protagonist (Sol Badguy) has not appeared yet
Haehyun (Guilty Gear) = Very Unlikely
Reason: Protagonist (Sol Badguy) has not appeared yet
Jam (Guilty Gear) = Very Unlikely
Reason: Protagonist (Sol Badguy) has not appeared yet
Dizzy (Guilty Gear) = Very Unlikely
Reason: Protagonist (Sol Badguy) has not appeared yet
BB Hood (Darkstalkers) = Very Unlikely
Reason: Protagonist (Morrigan) has not appeared yet
Orie (Under Night In Birth) = Very Unlikely
Reason: Protagonist (Hyde Kido) has not appeared yet
Hilda (Under Night In Birth) = Very Unlikely
Reason: Protagonist (Hyde Kido) has not appeared yet
Pauline (Super Mario) = Very Unlikely
Reason: Already a Spirit
Pyra (Xenoblade) = Very Unlikely
Reason: Already a Spirit
Reimu Hakurei (Touhou) = Somewhat Unlikely
Reason: Indie character
Kat (Gravity Rush) = Borderline Impossible
Reason: Sony owned series
Akira Howard (Astral Chain) = Medium Possibility
Reason: First party but still has a chance as Bayonetta 2 was also published by Nintendo. Potential Fighters Pass 5?
Octoling (Splatoon) = Very Unlikely
Reason: Already a Spirit
Pearl & Marina (Splatoon) = Very Unlikely
Reason: Already a Spirit
Twintelle (Arms) = Very Unlikely
Reason: Already a Spirit
Min-Min (ARMS) = Very Unlikely
Reason: Already a Spirit
Krystal (Star Fox) = Extremely Unlikely
Reason: Already an Assist Trophy
Ayumi Tachibana (Famicom Detective Club) = Very Unlikely
Reason: Already a Spirit
Mach Rider (Mach Rider) = Very Unlikely
Reason: Already a Spirit
NOTE: The gender of Mach Rider is left ambigious
Marina Liteyears (Mischief Makers) = Very Unlikely
Reason: Obscure series,
Arle Nadja (Puyo Puyo) = Likely
Reason: Highly requested, unrepresented series, no competition.
Alis Landale (Phantasy Star) = Unsure
Reason: The transistion of making Phantasy Star an online game has left her chances quite ambigious.
Alicia Melchiott (Valkyria Chronicles) = Very Unlikely
Reason: Protagonist (Calude Wallace) has not appeared yet
Amy Rose (Sonic the Hedgehog) = Very Unlikely
Reason: Already a Spirit
Roll (Mega Man) = Very Unlikely
Reason: Already a Spirit
Heather Mason (Silent Hill) = Somewhat Unlikely
Reason: Although a protagonist, she will have to compete with James Sunderland and Harry Mason as the Silent Hill rep
Katrielle Layton (Professor Layton) = Very Unlikely
Reason: Protagonist (Hershel Layton) has not appeared yet
Velvet Crowe (Tales of Series) = Somewhat Unlikely
Reason: Although a protagonist, she has very heavy competition with Lloyd Irving as the Tales rep.
KOS-MOS (Xenosaga) = Somewhat Unlikely
Reason: Protagonist (Shion Uzuki) has not appeared yet, however KOS-MOS is a major character
Athena Asamiya (Psycho Soldier) = Unlikely
Reason: Although a protagonist, the series is very obscure. Even with King of Fighters, she is not the protagonist of the series.
Saber (Type-MOON) = Very Unlikely
Reason: Protagonist (Shirou Emiya) has not appeared yet
Mythra (Xenoblade) = Very Unlikely
Reason: Already a Spirit
Ana (Earthbound) = Very Unlikely
Reason: Already a Spirit
Paula (Earthbound) = Very Unlikely
Reason: Already a Spirit
Adeleine (Kirby) = Unlikely
Reason: Although not a spirit, she is a first party character which gives her a low priority
Brittany (Pikmin) = Very Unlikely
Reason: Already a Spirit
Alex (Minecraft) = Balanced Possibility
Reason: If Steve becomes available, Alex will most definitely be one of his alts.
Ms. Pac-Man (Pac-Man) = Unlikely
Reason: Although not a Spirit, she is very much the same as Pac-Man in terms of abilities
Medusa (Kid Icarus) = Very Unlikely
Reason: Already a Spirit
The Sayonara Wild Hearts protagonist = Very Unlikely
Reason: Indie character, non-existent support
Ring Fit Trainer (Ring Fit Adventure) = Unlikely
Reason: Although she isn't a spirit and has a sudden increase in popularity, it seems too late to include her.
Ajna (Indivisible) = Unlikely
Reason: Indie character
Estelle Bright (Trails in the Sky) = Unlikely
Reason: Although the protagonist of the series, the character isn't very highly requested.
Daemon X pilot (Daemon X Machina) = Very Unlikely
Reason: Main characters are already spirits
Sophitia Alexandra (Soul Calibur) = Very Unlikely
Reason: Protagonists (Siegfried/Nightmare) haven't appeared in the game.
Anna (Fire Emblem) = Very Unlikely
Reason: Already a Spirit
Kasumi Yoshizawa (Persona) = Unlikely
Reason: Although not a spirit, her first appearance in Persona 5 the Royal would be too late for DLC.
Ayane (Ninja Gaiden/Dead or Alive) = Very Unlikely
Reason: Protagonist (Kasumi) hasn't appeared in the game.
Mym (Dragalia Lost) = Very Unlikely
Reason: Protagonist (Euden) hasn't appeared in the game.
Katalina (Grandblue Fantasy) = Very Unlikely
Reason: Protagonists (Gran and Djeeta) haven't appeared in the game.
Noel Vermillion (Blazblue) = Very Unlikely
Reason: Protagonist (Ragna the Bloodedge) hasn't appeared in the game.
Sakura Shinguji (Sakura Wars) = Very Unlikely
Reason: Although the protagonist of the game, not a popular fan request.
Kachi (Sin and Punishment) = Very Unlikely
Reason: Saki Amamiya and Isa Jo are both spirits
Jill Dozer (Drill Dozer) = Very Unlikely
Reason: Already a Spirit
Nina Williams (Tekken) = Very Unlikely
Reason: Protagonist (Jin Kazama/Heihachi Mishima) haven't appeared in the game (Heihachi is part of Pac-Man's taunt though)
Asuka (Tekken) = Very Unlikely
Reason: Protagonist (Jin Kazama/Heihachi Mishima) haven't appeared in the game (Heihachi is part of Pac-Man's taunt though)
Sami (Advance Wars) = Very Unlikely
Reason: Protagonist Andy is a spirit.
Kassandra (Assassin's Creed) = Very Unlikely
Reason: Although the main protagonist of Assassin's Creed Odyssey, Ubisoft seems to want to include Rayman first.
Aveline de Granpre (Assassin's Creed) = Very Unlikely
Reason: Although the main protagonist of Assassin's Creed Black Flag, Ubisoft seems to want to include Rayman first.
Tracer (Overwatch) = Somewhat Likely
Reason: Protagonist of the series but developer has spoken about her which lowers her chances
D. Va (Overwatch) = Unlikely
Reason: Protagonist (Tracer) hasn't appeared in the game
Mercy (Overwatch) = Unlikely
Reason: Protagonist (Tracer) hasn't appeared in the game
Monster Hunter (Monster Hunter) = Very Unlikely
Reason: Rathalos appears to represent the series as a whole.
Joanna Dark (Perfect Dark) = Very Unlikely
Reason: Although the main protagonist, she is not highly requested.
Shepard (Mass Effect) = Very Unlikely
Reason: No fan support
Phosphora (Kid Icarus) = Extremely Unlikely
Reason: Already an Assist Trophy
Lip (Panel de Pon) = Very Unlikely
Reason: Already a Spirit
Yuri Kozukata (Fatal Frame) = Extremely Unlikely
Reason: Already an Assist Trophy
Momiji (Ninja Gaiden) = Very Unlikely
Reason: Protagonist (Ryu Hayabusa) hasn't appeared in the game
Agitha (Legend of Zelda) = Very Unlikely
Reason: Already a Spirit
Toon Zelda (Legend of Zelda) = Very Unlikely
Reason: Already a Spirit
Medli (Legend of Zelda) = Very Unlikely
Reason: Already a Spirit
Marin (Legend of Zelda) = Very Unlikely
Reason: Already a Spirit
Jodie Summers (F-Zero) = Very Unlikely
Reason: Already a Spirit
Viridi (Kid Icarus) = Very Unlikely
Reason: Already a Spirit
Aigis (Persona) = Very Unlikely
Reason: Main character is a Mii costume
Coco Bandicoot (Crash Bandicoot) = Very Unlikely
Reason: Protagonist (Crash Bandicoot) has not appeared in the game
Junko Enoshima (Danganronpa) = Very Unlikely
Reason: She is not one of the protagonists of the series
Ada Wong (Resident Evil) = Very Unlikely
Reason: She is not one of the protagonists of the series
Agnes (Braverly Default) = Very Unlikely
Reason: Protagonist (Tiz Arrior) has not appeared in the game
Ruto (Legend of Zelda) = Very Unlikely
Reason: Already a Spirit
Gothitelle (Pokemon) = Very Unlikely
Reason: Although not a spirit, it originates from a generation past it's prime and is not highly requested.
Mipha (Legend of Zelda) = Very Unlikely
Reason: Already a Spirit
Urbosa (Legend of Zelda) = Very Unlikely
Reason: Already a Spirit
Linkle (Legend of Zelda) = Very Unlikely
Reason: Although not a spirit, Legend of Zelda characters other than Link, Zelda and Ganondorf are not reoccuring characters