The goal of this suggestion is to allow Blue Mage into more content without removing any of its current content or changing it significantly, instead allowing people to use their hard won spells in content other than the Carnival.
At max level you either do a quest or simply unlock three more options in the Blue mage spell book. These are titled Tank, Healer, and DPS, respectively. The first option remains the same, just Blue Mage. The original bar doesn't change. It can slot any spell and it has the same limitations as currently.
First bar is, for example, tank. The first twelve slots are all required spells, needing certain specific and a series of spell options. First spell is Aetheric Mimicry and changes to Tank upon selecting this bar. Next is Mighty Guard. Then from there a series of spells intended to fill in the rest of the role, things like Rehydration, Cactguard, Frog Legs, Toad Oil and the like. The damage slots here give you a number of options to choose from among the list of spells that have similar effects. The last slots are 'free' slots, allowing the Blue Mage to select few spells from a limited list to further customize the job.
This bar cannot be selected until you have the requisite spells. Upon selecting this bar, your role is changed to tank and you can que up in party finder up to content just before current progression. You cannot change the main skills after you select the role.
Healer and DPS function the same way. This allows the dev team to determine what spells can and can't be used, preventing the broken aspects of Blue Mage from being used while still allowing them to utilize their unique aspects in different roles. The free slots can be altered based on role so as to not allow them to be overpowered ((No strong healing on tanks, so on and so forth))
Overall, this maintains Blue Mage as it is now, using the early and even late levels to gather spells but now with the goal of being able to utilize them in more content, thus encouraging more people to do said old content. It also allows for less social Blue Mage players to unlock the DPS role later and then utilize that to unlock new spells, thus giving people more options.
All of the tech involved already exists in game. Switching to a new job switches your roles, spells are already limited based on levels, Blue Mage is already locked out of duty finder so restricting that access already exists, and all the spells to allow Blue mage to perform those roles already exist. The main reason I suggest semi-limited is because Blue Mage doesn't rely on a main weapon which is an automatic advantage which I think would be unfair in progression.
And of course the easier version is to just allow the DPS role for Blue Mage instead of all three but I feel like that would be a waste of potential. We have the ability to add a new Tank, Healer, and DPS all in one go and it'd be a shame to not capitalize on that