If I had to hazard a guess they're probably gonna take a cue from FFIV - The After Years at some point in the future since all other Threat-Level Villains has proven not capable of matching the WoL at this point.

So its likely maybe the Ivalice Raids are going to be required like how they turned Crystal Tower into a requirement.

I suspect its going to be a cosmic-threat villain this time involving the auracite, so maybe a mix of FFVII, FFIV- After Years & Tactics rolled into one.