Metal Face and Yellow Devil are two obvious ones. Im surprised that no legendary pokemon were made into bosses. Dialga, Palkia, Zekrom, Reshiram, even bring Rayquaza back from Subspace.

Majora for Young Link. Metal Gear for Snake. Metroid Queen for Zero Suit.

Sonic has a lot of options to choose from in this regard. It actually makes me disappointed to think about
- Perfect Chaos (SA1)
- Biolizard/Finalhazard (SA2)
- Metal Overlord (Heroes)
- Devil Doom, (Shadow the Hedgehog) this would have been perfect to have if Shadow were an echo fighter.
- Solaris ('06)
- Dark Gaia (Unleashed)
- Time Eater (Generations)

And then there's all the different mechs that Dr. Eggman has used like the Death Egg Robot, or the Egg Emperor, or the Egg Dragoon!

I didn't want it to be exhaustive! I would love to have Mother Brain in her final form from Super Metroid for Samus. Sephiroth for Cloud?

I would have liked them to use the assist trophies for some bosses, up their moves and have a health bar, like Rathalos, at that point you already have a functional model is just about programming it as a boss.

Fighting the Black Knight as Ike would have been an amazing treat to one of FE's most iconic moments, same applies to assists like Andross, Goroh and Mother Brain. Real shame about Metal Face, since he has like 2 hours worth of voice lines.

Include bosses for all characters. I dont want to give too many examples.

Giant Baby Bowser and Porky would be great! How about Ghirahim for Zelda/Shiek?