Also, promote Master Hand into playable character as he's actually made playable temporarily during WoL's true climax

Add sans deltarune because sans undertale is already an easter egg.

"im sans deltarune, sans undertale is my father"

lol

Make it impossible to unlock Waluigi without cheating/hacking.

Rearange the order of the character's to when they were added in the game so would come first.

Actually it is like this:
1-
2-
3-
4-

Can't find it but I remember cloud or someone from the dev team stating that was the frist to be put in the game but I could be wrong.

You're right. He was the first character programmed into the game.

Include a voice clip of Alpharad saying "FORWARD AERIA!L" everytime Mario's Fair successfully meteor smashes.
...wait, no! Include the voice clip everytime Mario's Fair lands.
...wait, no no! Include the voice clip everytime Mario uses his Fair.

Sonic should get instakilled when getting hit.
But Tails should never take damage because of his powers as a sidekick.

Everytime Tails "dies" he just fly back to the stage, like in Sonic games.

Make the Super Mushroom effect last until you get hit once and make the Poison Mushroom effect last until you get KO'd.

Add Mini Mushroom item
It shrinks anyone to just 1 pixel, smaller than poison mushroom's effect.
Those with Mini Mushroom effect gets the "Light" status from Special Smash, (high jump, floatier physics) but will be instantly KO'd if hit.

Replace Master Hand with Wham Bam Rock's hand.