Ultron would seem to be confirmed (or a Ultron/Sigma hybrid) as the final boss. Thanos has to be in given the Infinity Gems. Loki seems likely but not certain. Carnage would likely just be a swap fro Venom if he's in, assuming we get Eddie Venom and not Flash Venom. Thor is in, whether that's Odinson or Lady Thor or both. No way will Hyperion even be considered. Medusa seems likely, assuming that Ms Marvel and Quake aren't picked instead. No Odin I would assume. Onslaught is X-Men so he's out. Scarlet Witch is a debatable one given her MCU ties and Magneto ties being erased, but one I think might be pretty likely. No way is Doctor Strange not in. And Winter Soldier... hmmm, I would like him but they might have to prioritise.
I gather Ant Man and Groot have already been leaked as playable characters somewhere.