Lara Croft - single biggest star not yet in the game
Scorpion - Midway, classic American arcade representation
Doomguy - PC/FPS boom represenation
Tecmo-Koei - Ryu Hayabusa, Angelique, or Kasumi(ugh)
Technos Japan = Billy & Jimmy, Kunio-kun
Hudsonsoft - Bomberman
Taito - Bub & Bob, Kunoichi
Level-5 - Professor Layton, some Yo-Kai Watch garbage
Travis Touchdown
I think those are all the big 3rd party reps that still need to make it in. Bomberman is sadly probably out of the running since a third Konami-owned character would be probably go to Goemon. Goemon is awesome but I would prefer Hudson to get an actual rep instead of just an assist trophy.