Some ideas that I have had in the past that I think would be cool:

Omega + Eden Mount
Please.

Just let us into expedition content
Eureka and Bozja would both be awesome playgrounds for BLU. I know that balancing would be difficult or even impossible, but the zones are already not balanced well.. so I think it'd be fine. Additionally, Bozja especially is quite lacking in activity overall but if we allowed BLU enthusiasts in it would bring a lot of activity! And on this note..

Ultimates
Seriously, just let us do them. How can there be concerns about balance when GNB/DNC/SMN are allowed to do the level 70 Ultimates? UCOB has been 7 manned, cleared with all tanks, cleared with 100+ deaths.. the balance and prestige are already ruined. We already have the ability to check if a party is fully BLU, so just make a separate achievement and reward for 8 BLU and only reward the original achievement and totem if a group has 0 BLU. Even better just disallow entering the duty unless you have 0 BLU or 8 BLU.

Blue Mage Battle Royale
I had this idea a long time ago. It's basically a generic battle royale-style game with all BLUs. Everyone starts with Water Cannon and there are random (weak) monsters from which you can learn other abilities. Defeating another BLU gives you their spells, and so on. It's been mentioned as something the dev team had thought of as well, so I'm high on hopium that it will happen eventually. Obviously this would involve a monstrous amount of development work, though.

Collaborative Masked Carnivale Duties
These would need to be strictly optional, but imagine a boss-rush where you need to handle the boss levels either simultaneously (similar to Utimates) or just one after another. If the former, allowing for 2-4 BLUs to do it together would be great.

More interesting combat loop
Right now we Moon Flute to dump all our high potency spells and then mostly spam a filler spell for 2 minutes. It would be interesting to add more variety. I had some ideas in this video, but the TL;DW is that I'd like for there to be a reason for BLUs in a party to take different spells from one another.

Obviously I don't expect a ton of dev time/effort to go into making brand new things, but it feels like there is some obvious low-hanging fruit here for us.