I'm usually in the "more of what already works" crowd, but I do think a new game mode that incorporates more players could be fantastic. Maybe adding in two more factions for a more true 4-player experience could work.
It might be cool to see the Hutt Cartel and Shadow Collective added.
That way, you can preserve the main tension between the Rebels and Empire, but each criminal organization can have their own goals, and act as rivals to each other- adding aid to either side in the war in a way to try and one up each other. You may even sometimes see both criminal factions team up with either the Empire or Rebels together in an effort to try and win more favor than their competitors.
Balance would be less finely tuned, of course (due to the obvious aspect of how swingey the criminals can be) but in the right groups this could be a ton of fun. Kinda closer to the GOT board game.
Give how important the idea of game length is (as in Rebels want less rounds, Empire needs more) I think the main objectives of the original factions would also need to be altered. Maybe something closer to the original PC game, in that both sides have more clear cut objectives. Honestly, that would address a problem many fans had with the original game's endgame often anti-climax, anyway.