Add Duty Finder functionality to the Blue Mage Log, on top of the existing Party Finder stuff. Alternatively add a "Limited Job" setting to the Duty Finder settings that Blue Mages can use to queue for dungeons/etc with others who've set that setting.
Add a Daily Challenge: Blue Mage option to the Roulettes (why is it headed "Daily Challenges" when there's literally just the one for Frontlines? about time to add more). That gets more people doing Blue Mage instanced content via the Duty Finder daily, in turn making things like the weekly log easier and potentially learning spells (I queue solo for Tam-Tara to learn Mind Blast, 3 other Blue Mages use the Daily Challenge and get matched with me, for example).
Add a specific Blue Mage Relic armor and weapon, with rewards from the Daily Challenge and weekly Carnival/Log being required for earning items for various steps, on top of perhaps other concepts (something like the Animus books with a focus on open world content could work well). That, if nothing else, gives Blue Mage some longevity. That we could just buy the new weapon and accessories with Allied Seals, which I'd long since capped and run out of use for, is frankly pathetic. There could have been a whole process for earning those that had players engage with Blue Mage content, instead they were just nothing... Blue Mage honestly needed its own currency here, not to piggyback of an incredibly outdated Hunt currency that's over half a decade old...
That last point could be really interesting too, if you make the gear actually interesting from a gameplay perspective. As it stands the gear for Blue Mage is just carbon copies of old Caster gear, that's incredibly boring. If you slapped interesting Blue Mage specific stats onto the gear? Then going out of your way to obtain that gear essentially becomes the same as going out to obtain the spells (which I would argue is one of the high points for Blue Mage). Something like "Enhances "1,000 Needles" effect", for example. That's interesting. That makes you look at your spells in a new way after obtaining such gear. That's engaging. Perhaps instead of such stats on gear directly, having Materia for that could be a feasible way to implement such stats in a way that has more longevity... If the job has to be limited for balance reasons, you might as well get creative with it in these kinds of ways... Isn't that the point of making it limited? So you can try crazy things out with it? Why then settle for the exact same gearing system?