I think FC-exclusive content could fix this. SE did a great job making this a game where an FC is by no means required.

It used to be that only FCs could buy houses. Now an individual can.

You used to be able to use the FC airship to go into Diadem, but you could just as easily find a group in PF.

The problem with this is FCs are made more obsolete in a game they were barely relevant in.

My suggestion?
FC exclusive mini-games and dailies for rewards you cant get anywhere else. Give us bizarre guildhest-like instances with bonus objectives and where you can unlock/use crazy FC-Limit Breaks with varying effects like summoning a swarm of fat chocobo to aid you. The rewards could make overworld easier for a time like FC buffs, but better.