Paper Mario might be interesting as a fusion of G&W's mechanics and Mario's, but it's not as if the Mario set is underrepresented.
I've already spoken my peace on Geno and K. Rool.
Leon, Marx and Omega Metroid are all similarly redundant, i.e. there's no value in scraping the bottom of a franchise's barrel just to add another playable character from that set. If all of a franchise's worthwhile characters are already in, then so be it.
I think Captain Rainbow might be a little bit of a stretch.