As far as newcomers i cant be anymore content with what we have now, but if I were predict whats to come I would go with atleast SEVEN more characters to come for the base game rounding up to 72 charcaters total for the base game.
1) Chorus Kids (Rhythm Heaven Rep)
Starting off with this series is a little funny cause out of all my picks, I see this character to be the least likely to join Smash. What keeps this character on my list is the rumors of its inclusion being in the last title even with the "Gematsu" leak. Though a leak could mean anything, the concept of the character seemed interesting and even hearing of their data within the game led me to believe they were close to being developed. If the chorus kids were to be a grouped character to work similar as the "ice climbers" or even "pkmn trainer" I could see why the character may have been scraped due to the 3DS limitations. Though this is only my speculation I would love to them represented in Smash, especially with their recent 2015 title Rhythm Heaven Megamix for the 3DS. Their moveset could heavily rely on rhythm timed movement bringing unique techniques to the table. (Could even see Tibby since he's the protagonist in Megamix)
2) Decidueye (PKMN Gen 7 Rep):
Sun & Moon representation is already shown to be in game so it wouldnt be to farfetche'd to see something from the 2016 title. With now 9 PKMN characters represented in smash (7 if were talking slots) we have some popular new Alolan Pkmn to choose from. Lycanroc, Mimikyu and Incineroar seem to be a strong lead
as far as plausible picks but for me Decidueye takes the cake with his Grass/Ghost typing, unique possiblities in its moveset and final smash being its Spirit Shackle Z Move.
3) Spring Man (ARMS/Orginal Switch Title):
The 2017 title would well represent one of the Switch's new series and fits the theme of smash perfectly. His techniques and attacks could be creatively implemented, and his inclusion would add a nice modern day representation to smash ultimate.
4) Rex & Pyra (Xenoblade Chronicles):
A tough one to think about here (If we dont get two playable reps in this series). Elma from "Xenoblade Chronicles X" in the 2015 title, one the main characters in Chronicles X could provide a great addition to the xenoblade franchise. But i have both a weak and strong argument aganist tge characters inclusion if we were to choose ONE new rep. The weaker argument is that the players avatar was the true protagonist throughout the game compared to Xeno 2 where you play as Rex & Pyra, the stronger argument is Xenoblade 2 is very popular and with Ultimate coming to the switch and the game going to be atleast a year old by its release, I could see Rex getting implented especially since the game considered the 2nd in its series. (The alternative for Elma in my case would be her as an assist trophy).
5) Byleth (Fire Emblem Three Houses):
Byleth the apparent default name for the Fire Emblem Three Houses protaganist (*Could be totally wrong). In the possiblity Sakurai goes all out in this new title and we dont see DLC, i wouldnt be suprised if we see the protagonist from the new "2019 FE" title be included. Just like Roy and Corrin he would be a character put in without the charcters game of origin even being sold in stores (though Fates was out in Japan in 2015 but not the US). He would most likely end up the last character you battle in Allstar mode since he would be the newset among every other fighter.
6) Simon Belmont (Konami/Popular Classic):
The vampire slayer being hinted in recent leaks from "Vergeben", Snake returning as a veteran and Bomberman as an assist trophy only makes his inclusion seem more plausible and could be a great way to represent one of konamis greatest series that exceled on the nintendo back in the 80s. Naturally I could see them using his classic design other then Simons more recent appearences.
7) Rayman (Ubisoft Representaion):
From a trophy in the previous smash to being playable in the best iteration yet. His inclusion would honestly top the game off and the Rabbids as an assist trophy wouldnt hurt especially with their recent popularity with the Mario & Rabbids game
(*Pure Speculation and does not view what I actually expect to happen) If we were to get a reveal a month I would see something along the lines of this happening...
JUNE
Later this month after E3 or even early July Nintendo reveals Spring Man to be playable since ARMS released in June 16th 2017.
JULY
Chorus Kids (Rhythm Heaven Rep) are announced in a individual trailer or in early August at EVO to coincide with their DS release on July 31st 2009.
AUGUST
Rayman is revealed in a Nintendo Direct that lists titles coming Fall 2018 for either August or early September. Hes revealed at the end and coincides with his Rayman Legends Wii U releasing back in August 29th 2013.
SEPTEMBER
Rex & Pyra (Xenoblade Rep) are revealed to promote their DLC pack "The Golden Country" dropping September 14th 2018.
OCTOBER
Simon Belmont is revealed this month to go along with Super Castlevania IV releasing back on October 31st 1991. Might even be revealed in Nintendo Direct that reveals titles releasing in early 2019, or a actual Smash Ultimate Direct (Which could reveal two newcomers).
NOVEMBER
Decidueye (Gen 7 Rep) is revealed to go with Pokemon Sun & Moon releasing in November 16th 2016 or Lets Go Pikachu & Eevee on Nov. 18th 2018.
(Could even be part of a Smash Direct)
DECEMBER
Byleth (FE 2019 Rep) gets the final trailer and is revealed before launch or even a cahracter kept secret in game. I see him revealed in December due to FE 2019 being delayed to Spring 2019. (Yet another fighter that may be better off revealed in a Nintendo Direct, Smash Direct or even future DLC).
*As for anyone thats not on my list that has the biggest chances of throwing it off is King K. Rool, Impa/Tetra, Bandana Waddle Dee, Crash Bandicoot, Kingdom Hearts Sora, and possibly Banjo Kazooie due to popularity.
**Also as far as "Echo Fighters" though Sakurai did give these characters a label, I dont believe he opened the doors for several other characters to recieve echoes cause of two reasons. One, these characters (as far we know now) actaully take up roster slots and to have a buffet full of these echo fighters I feel would be disservice since the newcomer pool this time around is gonna be small, especially when we have a number characters that function "somewhat similar" to Echo Fighters but arent under that category (Young Link, Pichu, Roy etc.) Two, is that even with the last game he kept a small number of these echo characters to a minimum being Dark Pit, Lucina, and Dr. Mario (even though he's not one now he was still grouped with those characters im Smash 4). So whats to say our only Echo characters this time around will be Dark Pit, Lucina, and Daisy? (Other than possible DLC)
***If there were to be Echo Fighters I would go with 3 being Impa (Sheik), Chrom (Ike), and possibly Shadow (Sonic).