Soma would be our 21?! But we already
have a pink character that can absorb enemy abilities!
Nah, but Soma really would be cool. I was first introduced to Castlevania with an LP of Aria of Sorrow, so it'd be cool seeing him.
You could definitely do something with enemy souls. Soma doesn't change with souls, unlike Kirby, and most souls use the same animation, so it won't necessarily be
quite as difficult as doing Kirby again. For simplicity (and ease of development), it should probably just have the one type of soul, but I can't deny that having red, blue, and yellow souls per fighter would be fun.
Also, you could give him some souls from the game as well. I think you have to have the Flame Demon soul, the Giant Bat soul, and the Succubus soul all in there somewhere. And I feel like I'd want Student Witch in there as well. It always sticks in my head that you can just throw cats at people.