Was thinking about people's desire to keep Blue Mage learning from monsters but the valid worry that if you didn't learn certain spells people would find you severely gimped and annoying to play with.
Possible weird solution:
Blue Mage is split into brackets, level 1-10, 20-25, etc, etc. Blue Mage doesn't level the normal way but rather levels through obtaining abilities, he unlocks new brackets when he fills out the previous one. Some abilities coming from dungeons/trials to keep him wanting to not just solo the entire time. Would make his leveling pace a bit funky, depending on what he had to do - its possible he could level the fastest, or slowest, depending on bracket and what was in it. It could be controlled more so that certain abilities in the bracket come after others, if people wanted to emphasize certain abilities like capstone level 70 ability.
Personally I imagine learning less on chance this time around, like taking a hit vs watching it is worth certain amount of points, an ability that increases defense ('shadow skin' 'sentinel' like ability) could also increase learning points so if you timed it right even better, and perhaps dungeon/trial versions are worth more as well. So you just fill up the EXP bar of a specific ability to learn it, that bar being filled up granting EXP to your job. I don't think they'd give Blue Mage significantly more abilities than other jobs just because I think they'll try to keep it playable on a controller, or at least they'll use some sort of "class" or "slot" system to force a maximum at one time. I imagine just picking out iconic / interesting theme'd / use abilities instead than giving them all is what would happen (easier to balance, less clutter).
But the point of this idea, which could be mutated and refined, is that you couldn't join a level 70 dungeon without being level 70 and you couldn't be level 70 without having all the appropriate abilities. So when a dungeon has a minimum level requirement they're actually setting a minimum blue mage ability set requirement.
Still run around get abilities in the wild, it just determines your EXP now and you don't EXP the regular way. Of course there are many other ways you could do it too, just a thought.
Also aside on the Blue Morpho lol: morpher isn't a major class in FF games but I do like jobs that can change shape, would be fun imo - I like transformation jobs.
I think that is a significant mistake if he does that, not that implementing Blue Mage HAS to be done but that he would implement it as not a job that can function in game as other end-game jobs. If he wants a monster book he should just do that, but if he takes Blue Mage and turns it effectively into a non-DoW/DoM, I think that would piss off a lot of blue mage fans. Vs just making a book of monsters.
Unless i'm misunderstanding the meaning you were infering, like Blue Mage gets a monster book and is DoW/DoM - the book just lets him control his abilities. I'm understanding your statement as he means to make Blue Mage /only/ a mini-game and not a functioning class like Red, White, or Black Mage, in which case I think it'd be better he doesn't reference it to blue mage at all.