u/Ballack1991

I have a feeling Stronghold will be next. Heroes 5 added it in an expansion (which was a great expansion btw). With Schism being the 'snowy' faction instead of Tower, and Hive being the lava-based faction instead of Inferno, I think we'll have to wait a bit before we see those get added.

The other one I think is likely is Fortress. Swamp as an added biome alongside the faction seems appropriate.

Obviously they could release a completely new faction as well. But seeing as they have already released two completely new factions, and given that Grove is pretty different from both Sorceress (H2), Ramparts (H3) and Sylvan (H5), I don't see that as very likely at this point.