Just have him use his chaos powers in his final smash, problem solved kek
Consider that Final Smashes aren't such a common occurrence in battles. im not even gonna get into Competitive Play, but even Casually, players won't always get a Smash ball in a match, and because of that, you are just gonna be stuck with the base moveset as part of a Character's image
if you just relegate Shadow's Known Chaos Abilities to a Final Smash, You are just gonna give an impression of Shadow as just another Sonic. and while Sega does want to establish Shadow has being an equal of Sonic, they also want to Establish that Shadow is a DIFFERENT Character and that he isn't Sonic
Making shadow green and not incorporating his chaos abilities which he doesn't use in most playable appearances anyway are very different things, Shadow can act like himself without using that stuff. Plus they can still use those powers in a final smash as well as various effects (darkness on punches, teleporting for rolls etc) if having them is really necessary to sega.
"most playable appearances"
The only game to not ever use Shadow's Chaos abilities in gameplay was the Free DLC of Forces. even in the games where Shadow does play similairly to Sonic he would have some sort of Chaos abilities in gameplay
SA2: Multiplayer, you had access to Chaos Spear and Chaos Control
Sonic Heroes: Team Dark's Blast attack Uses Chaos Control, which freezes time for 5-10 seconds
Sonic Rivals 1 and 2 (this is a game where everyone is a Sonic Clone, Even Silver, Knuckles, etc): Chaos Control was Shadow's special abilitiy
and then all the other games Shadow plays substantially different from Sonic. Shadow the Hedgehog, Sonic 06, Sonic and the Black Knight, Sonic Chronicles, Sonic battle...
Include all the boss fights where he would have his Chaos abilities (hell even Sonic boom got this right)
hell even without the Chaos abilities, How Sonic fights in Smash doesn't really fight well with Shadow's Character. Animations like Side Smash Down Smash works with Sonic because of his more light hearted nature. Shadow is Described as a more Dark Hearted Ruthless Character, and just putting Sonic's animations on Shadow doesn't capture Shadow's more dark nature at all. Along with the fact that Shadow is known to be more heavy on the Chaos abilities when he fights then any other character, and that is why Shadow as an echo will not suffice.