Feels like a good time to throw out various ideas while things are slow at the moment.
Tower Defense
We've had leves and that one hest where the goal is to protect artefacts from enemy attackers. How about a more challenging endgame version of this concept which could be a new type of mega leve or something else unrelated. Lots of structures or artefacts to protect over a large area, and maybe some limited resources like being able to set exploding traps, reinforce walls or deploy squadron members to certain points. As with any new content, rewards should be unique and substantial to encourage people to keep playing for many months.
Survival
Just the same as many kinds of game have. Unlimited waves of increasingly difficult enemies flooding into an arena; see how long you can survive. Can have website with player rankings, just like deep dungeon and pvp have. If you survive very far the enemies would get borderline impossible so it's up to the ingenuity of players to push the limit of what is possible.
Special Fates
I want something to keep all of the field zones active. First off, the existing big fates like Odin and Coeurlregina could have new rewards such as Materia V in exchange for their reward tokens so you have a reason to keep doing them unlimited times. But that isn't enough fun because those fates don't pop often. So I would like a new kind of big fate for all/most zones which can be triggered frequently by players, such as complete 20 normal fates with a gold rating in that area. Then people spread the news that a big fate popped around linkshells like we do with hunts, and people come because it offers random rewards which have a chance to be something special, or again tokens for shop rewards. It doesn't force people to do normal fates, but it makes them mean something if you want to fill time waiting for something else you can clear fates to progress the zone towards a big fate pop.