Alright, I know this sounds dumb and it
will never happen for obvious reasons, but...
Yes, I know adding it as a playable fighter would destroy the purpose of having assist trophies in smash and it would be huge slap in the face for future fighters like Waluigi or Lyn, but hear me out...
Assist Trophy Jar debuted in Super Smash Brawl (2008) and it's purpose was to summon various video game characters that couldn't be playable (it's like Pokeball meets non-Pokemon video games overall) and they would do couple of attacks to opponents.
It appeared also in SSB4 and Ultimate, but with new assists (because some assist characters got promoted into playable).
"But wait, how it could work since it's just a tiny humanoid trapped inside a jar??"
Well, as a fighter, the "humanoid" would "break" it's limbs out of the glass cage and they would look so monstrous (this means it's arms and legs look demonic due to having buffy arms and claws).
It's moveset could use assist characters' attacks, ranging from normal attacks to final smash.
Here's the example for it's specials:
Neutral Special: Tile spit - The jar morphs into Andross and spits the tiles in front of it.
Side Special: Kapp'N bus - It morphs into bus and drives straight at opponents like Wario Bike. It can even kamikaze itself when trapping an opponent inside the bus.
Up Special: Metroid float - It could fly for while and while pressing A, it can leech on opponents like Metroid AT.
Down Special: Bomb spawn - The jar morphs into Bomberman and spawns bombs, on ground and in the air.
Final Smash: Assist Trophy Throwdown - The jar explodes and causes all assists to come and wreck havoc on the stage, causing the players to suffer the getting beaten up by assist trophies.
It would be a middleweight mix-up fighter with no additional movement options.
The alts may be the issue, but the "ghost" could be colored in 7 colors while jar is untouched by alt colorization.
Like I said, he would be an unlikely inclusion for the game, but since we haven't got a proper playable Smash rep (Miis don't count), then Assist Trophy jar could work as fighter.