I'd love to see Dark Pit in the game as an alternate costume for Pit, complete with original VA and lines.
As for the other characters, I don't think Viridi really warrants a spot as a playable character given we never actually see her fight so her stage cameo works fine. I think Palutena is the most deserving newcomer from the Kid Icarus series, and as far as runner ups I would take Magnus first given he already has an established fighting style. Not sure how Medusa or Hades would translate given their massive scale. I know the "too big" argument is a bit of a joke, and Ridley in theory sounds like he could be scaled down to work but I can't say the same about those two.