Possible bigs:
The white ISD from Rogue One, which would include Thrawn, and the MC-75. They're big ships after all, and Thawn in the game would be considered "big" by many fans.
Or
The first Epic for Armada, which would likely be the Resurgent Class. Though I'm fine with seeing that, I'm hoping FFG waits until after Episode IX when there'll be the "not battle of Endor" big fleets fight. Still, I wouldn't rule out the Resurgent as the first Epic. Actually, I'd prefer them to release it first so that they can see how well it performs before the one we truly want to see, and see in all it's game changing glory.
Or
Super Star Destroyer!!! I suspect if they had meant this they would have said "something huge". If an ISD is 8" this thing should be bare minimum 16". Even then it would probably look too small. I think between 20" and 24" should be the size at least.
I'm personally hoping that the Rogue One bigs are announced alongside the first Epic. That Epic being the SSD.
Save the sequel trilogy stuff for Episode IX.