The only reason I don't think it will be Separatist is because there's hardly any character development of GG or Dooku. Yeah GR vs. Seps makes the most sense faction wise but thinking about the actual characters, those we have multiple of we have for a reason, to either show character development, like kylo and rey, or depict characters during different timeframes. We have 3 kylos to show his evolution from a general/warrior to the head of the FO. I don't think I need to explain Luke's character development. And kenobi, we have the strategic general of the galactic republic, old-man kenobi, and are now getting the jedi master version of him.
If we got GG or Dooku, how would they be fundamentally any different than who we already have in game? GG is already a general, I mean it's in his name and he has a cap ship. Someone mentioned "supreme commander GG" and I have absolutely no idea what that even means. Dooku? Well he gets his head chopped clean off so unless we start resurrecting characters and Dooku holds his head like a bad Halloween costume, that doesn't make much sense either. Finally, separatists (GG really) would make more sense if the GL version of Kenobi was strategic general. But it's not. It's jedi master and jedi master kenobi's greatest and most tragic adversary during this timeframe happened to be his padawan.
My opinion, the only version of a DS character that we don't have from this era yet worthy of GL status is Anakin as he embraces the dark side. JKA is the version of him as, well, a jedi knight. Duh. Vader is post transition, well established Vader. A new version of him would bring that iconic transition explicitly to the game. I know it doesn't fit the faction cadence particularly well and just considering factions, GR vs. Seps makes the most sense. Unfortunately, there isn't a realistic version of a Separatist remaining worth GL status.