On a similar note, any other veterans you guys want reworked for similar reasons?
So late to this but...
I'd indeed change Mario by removing FLUDD for the original Mario Tornado and make Down B either the Ground Pound or Goomba Stomp attack. Also boost recovery to Smash 64 levels. And make Fire Ball like the 64 version but fast like it currently is.
With Samus I'd make Charge Shot function much like it does in Metroid. The fully charged shot is weaker, but more spammable and charges faster. Uncharged shots are relatively more usable. Also Side B triggers Missile Mode which fires Missiles as quickly as the beam attack. Hold Side B to shoot a Super Missile, Down B is now Morph Ball and works like everyone else says it should work. I also would include the Melee Counter somewhere. Possibly U Smash or U Tilt. Screw Attack as N Air attack while freeing up Up B is also interesting, but not sure if I'd want it.
With DK, I want a size reduce except for his arms. Make him a little lighter, and faster and make him more of a heavy weight semi rushdown grappler. Keep the explosive KO power tho! Remove Headbutt for either a barrel throw or a grappling command grab attack. Make Handslap come out significantly faster and give him super armor on the move. This will make his archetype prosper. I'm also a fan of removing Headbutt and putting Spinning Kong there and make Up B grounded the barrel throw and off stage a Barrel Cannon.
With Diddy I want him to be less defensive and more offensive and semi rush down too. Buff F Air to it's Smash 4 self. Faster F Tilt, maybe F tilt with a tailslap like Mewtwo always had as a reference to DK64. Give him back his old range and better air speed. Not even fully against removing the Banana if they'd give him a sort of better option for offensive game play. As a sort of cartwheel version of Sonic's neutral B. However the Banana is staple and fine as it is.
I'd also probably wanna change both Lucina and Chrom. Give them their own moves which they share with each other, not with Marth and Roy. They can still borrow from them however.
Mewtwo I would also change up. Confusion should be changed into a ranged command grab with Psychic, which deals good damage and KO power as well as reflective properties. Something telekinesis should resemble. Disable I'd also remove for Psycho Cut or Psystrike. He can be overtuned in the Specials. Everything with the tail I'd replace with attacks that are energy based but ranged and disjointed. Think F Air and his Smash attacks, but more ranged and more disjointed. Also, more weight. Definitely 90+, ideally around 100 even. Mewtwo can afford to be more of a powerhouse character, as long as he's technically hard to play. I am also not against Mewtwo being more like his Pokken self.
Lucario I'd change around too. No more Aura mechanic. Make him more a balanced rushdown fighter. And give him his "Aura staff" thing. This could allow for way more range on attacks. Buff his attack power to be around 75% damage as he is right now. And buff his speed too. I think this character is honestly way undertuned and has potential to be way better represented and be a overall more fun character to play. Think of the Pokken version of Lucario basically.
Kirby I'd also change. Make him like the 64 version but less oppressive. More air speed, more range, better Smash attacks, Copy doing more like how it works in the Kirby games mostly. Kirby should be able to eat up non energy projectiles for example, be able to spit them back at his enemies. Up B I would simply make it to be the Warp Star cause WHY NOT!? Not the biggest Kirby expert but I do think there's way, WAY more to work with. He's a popular character after all, don't make him un fun to play.
Ness I would also change a little. U Smash and D Smash are becoming energy based attacks like Lucas. U Tilt and D Tilt are now the Yo Yo. And F Tilt is also a bat swing attack, like his attack with a battering item like Homerun Bat or Beam Sword.
Charizard I also want to make more aerial based. Better air speed and jumps for starts. That would allow for less weight cause that would boost his survivability a lot. If a solo character, down B becoming Fire Blast is a cute idea. Long ranged powerful fire attack. Boost his recovery. Make Flare Blitz more like Dragon Rush, the Custom Move of Smash 4. Or make Up B function like that, but be able to tilt the movement. I also definitely want something of an aerial Flamethrower. Also, speaking of that move, it's really bad. Either more range or devastating damage on impact or it's not worth it.