I, delusionally, want them to revamp campaign and make keeping territories accessible important to content progression. I also want them to make AOE campaign WS available to every job. This becomes the new place to farm exemplar points.
Their execution of this would be catastrophically bad and I know in reality it would be a terrible idea.
Campaign is, on paper, the single coolest thing they ever added to the game. It gave something for every job and every craft at every level. Everyone could contribute, and it all felt very meaningful. Fighting alongside big armies always felt really fun.
The problems were how unbalanced it was, how awful "Offensive" battles felt just whacking away for 1 damage on a tower that doesn't even defend itself, and how quickly irrelevant it became. And in true FFXI fashion, it was (and still is) pretty confusing.
But someone could take the good ideas and probably quite easily polish the bad ones and turn it into a whole game in itself.
For other endgame content, you could even reduce the efficacy of mercing by tying rewards to participation and performance, similar to how Campaign Battles operate. You just need to solve the abuseable, spammable means to max out your rewards. That way you can't just pay someone else to do everything on your behalf.