Does not account for taunts, victory animations, and the simple "this move now deals less damage" stuff you'd see in balance patches. Made within the context of a casual game.
Mario:
-Happier disposition
-Cape is swapped for tanooki tail. Functionally the same
-Mario Tornado has been restored, now with Cappy floating around as a disjoint. Mario himself is a sweetspot however, so Cappy functions as a damageless get-away tool like FLUDD
-Dash attack is the SM64 tackle
Donkey Kong:
-Happier disposition
-Classic Voice
-Headbutt involves DK slamming a barrel. Functionally the same.
-Down Smash and Down B’s animations are swapped - the new Down Smash can still be held as it could be as a special when less than half charged. Down B shares a charging resource with Giant Punch.
-Konga Beat is back
Link
-Skyward Sword/MK8 design.
-Classic bomb and hook shot restored
-Jab laser removed.
-New Final Smash: Heroes Across Time - TP, BOTW, and OOT Link assist as spirits.
Samus
-Some moves have ice properties. Purely cosmetic
-ZSS stance change restored to Final Smash.
Dark Samus
-Smash Attacks are now disjoints, with the assist trophy attack being used as a D-Smash.
-Laser homes and missile goes straight forwards
Yoshi
-Horse-like posture restored
-New Final Smash: Super Happy Tree - Yoshi eats fruit from the Super Happy Tree and his health is changed to -200%
Kirby
-Slide kick down tilt
-Air movement is closer to Jigglypuff
-Chef Kirby restored as Final Smash
-Some copy moves are side, up, or down specials
-Can get a random copy with an uninterrupted taunt
-Some copies aesthetically redesigned, such as Peach having a wig
Fox
-Shine restored to Melee functionality
-Landmaster restored as Final Smash
Pikachu
-Unmodified
Luigi
-Negative Zone restored
-Poltergust removed from grabs
-RNG powerful variants added to Luigi Tornado and Fireball - Fireball’s RNG variant summons a ghost who drags anyone in its path, while Tornado’s goes to the maximum height automatically
Ness
-PK Fire now has a use meter. Like wonderwing, it cannot be recharged unless you’re KO’d.
-New Final Smash: PSI Rockin’. Similar to Shadow Mario Paint. Jeff is involved.
-Smash 64 kick nair restored
Captain Falcon
-Unmodified
Jigglypuff
-Unmodified
Peach
-Peach Bomber rethemed to P-Wing
-Forward tilt is Peach creating magic sparkles with her hand, a la Fantasia Mickey
-Back air no longer has heart effect
-Toad replaced with Toadette for default costume, all other costumes have different-coloured male Toads.
Daisy
-All changes to Peach apply to Daisy
-Toad replaced with Gao, who breathes fire.
-Turnip replaced with carrot
Bowser
-Goofier disposition
-Proper voice
-Melee run animation implemented as dodge animation
-Goes into shell for crouch.
-Down B and down air animations swapped, Down B turns Bowser blue to reference the Blue Shell and ends with a cosmetic explosion.
-New Final Smash: Fury Bowser. Functionally the same as Brawl Giga Bowser
Ice Climbers
-Nana now purely copies Popo’s actions instead of being a unique CPU that follows.
Sheik
-OOT Design
Zelda
-TP Design and associated personality (Ultimate FS kept)
Dr. Mario
-Represents Dr. Mario as a series instead of Mario.
-No changes made to Mario apply to Dr.
-Angst and anger increased slightly
Pichu
-New Final Smash: My Pokemon Ranch - sends a foe into My Pokemon Ranch, where they are greeted by many playful Pichus, who accidentally zap them. Each character gets a Pokemon Ranch model
-Sleeping crouch restored
Falco
-Restored to Melee iteration completely
-Arwing is still FS, but controllable instead of cutscene
Marth
-Unchanged
Lucina
-Has tipper, now a full Marth clone.
Young Link
-Unchanged
Ganondorf:
-Has float
Mewtwo:
-Unchanged
Roy
-Unchanged
Chrom
-Unchanged
Mr. Game & Watch
-Down throw is based on Mario’s Cement Factory, with G&W turning a foe into cement; forward throw is Donkey Kong Hockey with G&W turning into DK; up throw is based on Egg, with the wolf throwing a foe up and down using his basket; back throw is unchanged.
-Changes forms for all moves, even Mario character moves
-New jab: turning into Link from Zelda and slashing
-Stanley is now F-Smash
Meta Knight
-Unchanged
Pit
-Unchanged
Dark Pit
-Unchanged
Zero Suit Samus
-Down throw sped up.
-Stance swap implemented in Final Smash
Wario
-Wario-Man FS from Brawl restored
-Down throw is a shoulder drop
-Animation has more gross-out
-Down Smash has Wario propelled by his farts
-Down air is a ground pound stall and fall, with a fart.
-Forward Smash and down tilt have Wario pick a visibly-rendered booger before doing so.
-Fart meter is signified by a visible fog coming out of Wario’s butt
-Wario burps when releasing foes from chomp
Snake
-Brawl FS restored
Ike
-Unchanged
Pokemon Trainer
-Pokemon doesn’t swap when KO’d
-Charizard Rock Smash implemented as side smash
-Can press back + down B to swap to the other Pokemon
-New Final Smash, now features a random selection from 6 of the original 151 Pokemon alongside the trainer 3. All original 151, including Jigglypuff and Pikachu, are featured. Also uses GB text instead of 3DS
Diddy Kong
-Canon voice
Lucas
-Duster added to Final Smash
Sonic
-Complete moveset revamp
-Uses classic design with Greg Martin mohawk.
King Dedede
-Brawl Final Smash restored
-Waddle Dee Toss restored
Olimar
-Has a voice
Lucario
-Takes small recoil damage in some moves.
ROB
-Stack-Up directional voice clips used for throws
-Hector in background like Pokemon Trainer
Toon Link
-Unchanged
Wolf
-Has landmaster
Villager
-Unchanged
Mega Man
-Completely reworked
Wii Fit Trainer
-Unchanged
Rosalina & Luma
-Luma colour is based on alt
Little Mac
-Significantly better air fighter
Greninja
-Unchanged
Miis
-Unchanged
Palutena
-Heavenly Light implemented as Up Smash, still deals no knockback.
Pac-Man
-Butt-Bash down air, using the pizza transformation
-Down Smash summons Pooka & Fygar, Up Smash summons Gyaraga, and side smash summons Mappy.
-Turns into US cabinet art design for Up B
Robin
-Meter removed completely
Shulk
-Future Connected design by default
Bowser Jr.
-Unchanged
Duck Hunt
-Mr. Stevenson and Tin Star added as extra rare Wild Gunman, both most and equally powerful.
-Can becomes weaker with every shot.
Ryu
-Input commands removed
-1v1 directional removed
Ken
-Input commands removed
-1v1 directional removed
Cloud
-Limit can only charge from Down B
Corrin
-Unchanged
Bayonetta
-Unchanged
Inkling
-Unchanged
Ridley
-Bigger
Simon
-Completely reworked
Richter
-Clone of the new Simon
King K. Rool
-Unchanged
Isabelle
-Unchanged
Incineroar
-Unchanged
Piranha Plant
-Unchanged
Joker
-Arsene charges from attacking instead of being attacked, teammate charge is still comeback based.
Hero
-Command Selection replaced with Snooze
-Erdrick is default
-Critical Hits reworked: instead of RNG, they appear only on Smash Attacks if performed out of a long combo.
Banjo & Kazooie:
-Given Nuts & Bolts design as default to better represent Xbox
-Uses the Mumbo Wrench in forward tilt.
-Throws are all Mumbo Wrench based. No tether grab however.
-New Final Smash: Bolt Bucket. Nuts & Bolts inspired, with a car ramming into foes.
Terry
-Command inputs removed
-1v1 directional removed
Byleth
-Unchanged
Min Min
-Completely reworked
Steve
-Completely reworked
Sephiroth
-Unchanged
Pyra/Mythra
-Has a brand new Smash-specific design, similar to Pit.
Kazuya
-Completely reworked
Sora
-Unchanged
Given the surprisingly vast considerations and hoops you'd probably need to jump through, even if you put aside characters from licensed properties who are just treated as memes on their own (like, say, Crazy Frog, Robbie Rotten, etc), you still have a lot of potentially difficult questions to ask on even an individual basis.
Admittedly, some would be easier than others - for instance, I'm like 90% certain anonymously-created stuff like wojaks or whatnot tend to automatically end up as free use, and then there's characters like the Crewmate, Slenderman or Lo-Fi Girl who have pretty obvious creators/rights-holders you'd approach to request permission like I mentioned earlier.
But like, would including Anonymous need the permission of DC due to the use of iconography heavily inspired by V for Vendetta in the "character design", or would Gigachad need the permission of Khalimov himself (the subject of the photograph) as well as Sudmalis for her editing and posting of them? Alternately, is "Skibidi Toilet" something you'd need the creator's permission for due to the argument of fair use, or would you exclusively need Valve's permission, given it's made in the Source engine and using Half-Life assets?
Even then, it'd be interesting to see where it'd be taken, and if it could actually be handled without looking like a LAWL roster. I guess the difficulty would obviously be that the memescape of the modern internet is so fast-moving that all of your characters and content would end up being really dated regardless of how you handled it. Like, "if you don't have Among Us Twerking Gif is it even a real game" followed immediately by "that meme died in 2021, where is Shrek Wazaouski"