Not that it's not almost screaming what it is. I see only a few options

First is a beastmen set including giddeus, ghelsba, palbough, davoi etc. T1 will be past/present stronghold T2 the lesser beastmen nations T3 Northlands with Shadowlord 3.0 as the big boss. CoP/Zilart/Nation/ToAU and most past areas are spoken for.

Option 2 a single final boss culprit of the whole invasion. (shinryu style)

Option 3 is a massing of deadly rivals shadowlord, odin, alexander, promathia all from this void world.

I can't think of any other finale unless they go back over areas. Personally I've expected northlands as finale since day one. After all in the end our true enemy is/was/shall be the shadowlord ever since the game's launch. (I'm leaving out the spoiler on purpose btw for those thinking I overlooked it) Though the boss rush with 2.0 enemies would be fun, then again I'm also still holding out for the story fight rematch feature. Anyway those are my predictions now all that's left is to see if SE is as predictable as I think they are.