SMASH 150 - 123 - MEDUSA

Technically not a Nintendo character at all, she belongs to the Greek Mythology franchise.

Medusa in Smash
Still my most anticipated new Kid Icarus character: professorfandango.deviantart.c… I think she could bring a lot of interesting new attacks to the series and she's a great villain (and we still need more villains!)
Some might argue that they don't like her new goth girl look. I quite like it, but am glad she can still turn monstrous when necessary.
I think she can still be classes as the series villain too, she was the original final boss and marked the beginning of the 'reset of the game' in Uprising. She even appeared right at the very end in a kind of contrived manner - it was a bit of a shame she didn't stick around a little bit more.

Plus she would work as a good antithesis for Palutena, given the Goddess of Light/Queen of the Underworld monikers. Some have suggested Hades as a new character, since he was the true final boss in Uprising and it is kind of alluded to the fact that Medusa was subservient to him even in the first game. I don't really like Hades but I can admit that he'd have a decent chance and would be suitable, though I don't see how he can possibly be more interesting than a head full of snakes.

Medusa in Kid Icarus: Uprising
It was very cool that the game was built up to suggest that Medusa was the final boss then after you beat her there's another stack of chapters still to go! It's a shame the rest of the game was so lame (the Space Pirates I could just about handle but cheap polygon aliens? Get outta here!) and I was very disappointed when Medusa showed up right at the end to 'hold Hades back' while you finish him off, and then dying again.
It would have been nice to see her ultimately overthrow him and become the new, true Underworld leader.

In fact, I don't recall all the details. Would Medusa even return in a new game? Isn't it implied (or explicitly stated) that Hades can never die because every defeat would just send him back to the Underworld? Surely the same would be true of Medusa? Well anyway in a new Kid Icarus game I really hope she returns in a good role.