That would work. A Switch version of 4 or Smash 5 could add assist trophies for the DLC guest characters that didn't get one. Add Tifa or Barret for Cloud, Chun-Li for Ryu, and Rodan for Bayo.
This could be a coincidence, but all the third party assist trophies so far have been rival/anatagonistic figures. Instead of Roll, Tails, and Ms. Pac-Man, we got Elec Man, Shadow and the ghosts. Metal Gear had Grey Fox back in Brawl.
With that in mind, I could see Akuma being Street Fighter's assist trophy, and either Sephiroth or a reoccuring enemy type like a Cactuar for Final Fantasy. For Bayonetta, I guess Balder could work?