Most Of The Originals Are Back It Appears
It seems like all of the playables are back in WFC2 except Warpath, Air Raid, Silverbolt, Breakdown and Barricade.* And DLC characters Demolisher, Onslaught and Scattershot. Obviously Zeta Prime won't be returning. And they've added some characters like Prowl, Wheeljack, Swindle and Mixmaster. It will be interesting to see what the final lineup will look like and if anyone else is added or if anyone listed here is removed. I think we'll see Silverbolt and Air Raid added back since the Cons still have 3 fliers. Warpath I think will be DLC because I think so many people will want to use him in multiplayer but I could be wrong. Anyways, I can't wait for the sequel and I hope that it's even better than the first.
*- Clearly I'm just going by what's been posted as of the date of this thread and obviously IMDB isn't the official source for this so this asterisk is for those that will most certainly correct me should this info change.