that's for

Replace every Mario's costumes into his bootleg and hacked ROM counterparts. Ex: Gran dad, Somari and why not, the recently diagonal Mario.

Don't forget the vinesauce Mario brothers like Genario and clown vomyot Mario

As for peach she gets the cursed Peach from the corruption streams - A completely magenta dress, pitch-black hair and soulless black eyes, and half of the face covered by shadows

this but dullvivid's version

make Waluigi's shield cover the entire screen

and this one was more for fun:
Remove these characters' shield bubbles
Ryu and Goku - they now simply block and super-armor through everything
Link - he'll use his actual shield and it has infinite durability, but cannot powershield (it ain't BoTW link)
Ness - replace with PSI Shield, which gives him heavy armor for a very short period of time
Kirby - he instead just becomes plain absolutely invulnerable like in the actual kirby games (at least some of them i guess)
Megaman - bring back proto shield

Keep the first part of Waluigi's Final Smash being the crossed out smash ball denying him a Final Smash, but then have him go into Power of Rejection mode out of rage and do several screen covering Tabuu off-wave attacks (while T-posing) before calming down.

But instead of actually T-Posing, Waluigi instead gives it his own twist to it:
This frame of his NAir but with his arms also extended to the sides

So, GoldenEye sideways H pose


But when the waves connect, he gives you a flying kick sending you to a boxing ring, turns into Rambo and shoots you more than 9000 times, then executes Goku's Meteor Combination, finishes by throwing a Bob-omb, which throws you into space, then Waluigi gives you a Roundhouse Kick in the face and grabs you and smashes you on the planet destroying everything completely. Deals 1000% damage (although the limit would be 999%).

This also inexplicably takes all of your stocks instead of only one and ends the game with Waluigi winning. Even in no-stock time matches.

If Naruto is in his Boruto recolor, replace his voice clips with Boruto's, make him throw his Rasengan and...
y'know what? just replace the Boruto recolor with literally Boruto himself.

Introduce echo fighters like these that are literally just palette swaps with different names and voice clips. (No need to change move properties like throwing Rasengan either)

If y'all are going to make this stuff very reliable, then just add a single character who can be "the overwhelmingly chosen echo fighter come from the dark dawn" now then the hypothetical character can simply copy the desired character's archetype but he can bosst their advantage at 5.21e+67% which means you can just win by hitting one single button (a simple tip, just press forward and you'll crash the game), and no other "clone character" is needed whatsoever.

give Lloyd a "retro" costume where it's literally just his SSF1 sprites

Actually, just make it his default costume

Give Master Hand a Final Smash.
His FS is literally the Master Core boss fight.

Make Simon's final smash able to turn all the characters into actual Christians.