Alright so;

All the expansions so far have been one word related to the main theme. (Heavensward about visiting the sky areas, the Heaven's Ward, etc. Stormblood about the battles going on, Rhalgr being brought up a lot, etc)

If 5.0 is going to be about a war against Garlemald then it'd have to be related to the major themes.

I'm gonna guess something like Steelreign or Decadence.