The best one is Hades or Medusa. Ill take Medusa as she is the only main villain who is female. Also i thinl Hades would make for a Masterhand/tabuu boss character in the single player mode

Shadow or Akuma could both be in. Wildcard pick, Tom Nook!

Eggman, Team Rocket pokemon trio, Skull Kid.

I'd love to see Bald Bull or Super Macho Man! That'd be absolutely unexpected

Akuma would be an easy echo fighter.

Personally I would prefer as someone stated above Hades or Medusa, Dark Pitt or Pitto as he is often called is more Meta Knight style rival but at this point so is Star Wolf and King Dedede.

Would like a Kirby villain. While most are really tricky due to form and size, maybe base Marx, Magolor or swordmaster Dark Matter.

Sephiroth, but he would require his own moveset

Akuma, easy Echo Fighter for Ryu (alongside Ken)

Eggplant Wizard, but would require his own moveset

Giovanni, but would require his own moveset (could switch between Persian, Nidoqueen, and Nidoking like Pokemon Trainer)

Bass, could be an Echo Fighter for Mega Man

Ghirahim would've been perfect. He would've been a new Zelda representative. Have no clue why they so stingy on Zelda characters.

Love these suggestions especially the Giovanni one. 

Hades bro, 4th wall troll who can resurrect the dead :p

Angry Sun blazes the competition!