One thing I think Smash should nab from PSASBR is how it handled menu music - you would boot up the game, and a random character would be on the main menu, and there would be a theme song variant that suited that character's vibe - dark and foreboding for Kratos or Radec, or peppy and upbeat for PaRappa or Sackboy. There were 5 themes, each assigned to a set of characters.
I actually this would work EXTREMELY well with Smash's orchestral music because orchestra, being so old, has so many variations and uses. You could keep the regular Smash music for Link or Ryu, but then have a whimsical Disney sounding Lifelight for the likes of Mario and Pikachu; a horror/Night on Bald Mountain sound for Ridley or Ganondorf; an action movie sound for Snake or Cloud; and a Can-Can style frantic sound for someone like Piranha Plant or Pac-Man. Would also be really neat with My Music.
Every Sonic character vs 1 billion dogs