Sort of Terra Branford and that sounds awesome haha. I would suggest that you jump ship on the primal bit, just to avoid summoner's toes and choose perhaps something very similar but fundamentally different (just need to get that one bullet point that means the world lol). Also it would help to define a strict list in comparison to the nebulous term primal, so rather than "transforms into awesome primals" its like "transforms into this very specific group of things that is limited to these items until I say otherwise".
Even if it still relates to primals which is fine I think it just needs to have some sort of special qualifier that narrows the list down to something unique as to why it is restricted - as it will have to be (start out the gate with something compelling but with just large enough opening for SE to add more an expansion later without retcon).
It will help a lot with expectations I think, and help drive a clearer story - so we don't get "why can't I have Garuda wings, Ifrit claws, Titan legs, and a Bahamut Nose!? lol".
I personally like seeing jobs that can transform, entirely or bits and bobs - I also think its a fun way to have that high adaptability in jobs like Summoner but within yourself rather than within pet reliance (not to be "better" than summoner, but different - that adaptability is a pretty nice feature I wouldn't mind seeing else where).
Also read the document, I always like dark / costly magic ideas - loved in FFXI and of course I liked reading it here (aether poisoning). (In real life my opinion shifts, be smart yo! lol but games are not real.. I think? :3). Btw what is a Brynhildr? Looked up on google, got a bunch of anime girls lol.
I don't think it has to be a Blue Mage either - I mean to say that I like the idea of transforming pieces of yourself or the entirety of yourself and /would/ like to see a job based on this but I also don't think it has to be called Blue Mage. Also it might be interesting to add that I believe there is already in game lore on a group of people who fuse monster parts on themselves.. could start from there too for an in game jump start.
Partly want to save the Blue Mage just because Raubahn - who at least at the moment has shown no transformation skills (but did rip from Ifrit a bit). For those who didn't know his decent Blue Mage allusions he has the same name for FFXI's Blue Mage job teacher, uses the same Blue Mage exclusive weapon (Tizona, exclusive to FFXI Blu Mages), uses abilities from Ifrit (and a few animal attacks), is referenced to an animal (bull), fairly impressive yell/beast rage (though you could call that "warrior" too), created an organization with the same name with similar purposes (protect the queen) where the blue mages in FFXI are known as the Immortal Lions and FFXIV's the Immortal Flames, and has said in different ways on multiple occasions that battle is the best teacher (not a unique idea to Blue Mage but screams Blue Mage mechanics).. I really want to see Raubahn become a part of the Blue Mage storyline lol.
People's concern is that you get a Blue Mage who didn't get X Y Z abilities because they haven't gone there yet. Or w.e, and it is a real concern. My suggestion is to completly change how Blue Mage levels to make it work without real concern:
For example you have a bracket that is formed around the dungeon requirements, SE looks and sees they have a dungeon open up at level 25 and they have these skills they'd like tanks to know, so they look into the Blue Mage ability list and place the abilities before level 25 and then in order to level you basically learn abilities. Your experience is your abilities, you go throughout the world, far and wide, to catch them all. . and as you learn the abilities you gain EXP. You cannot level without having the specific ability, therefore you will never see a Blue Mage in that level 25 dungeon that doesn't have "x defense skill". Do the same for end game content, in order to reach level 70 you need this list of abilities. To add SE can have more control and do things such as perhaps you /can/ level past that point but you don't unlock "30 and below duty finder" until you get that list of abilities. So you have a bit of control on how it works, but the important thing being is SE can make sure the issue everyone is worried about never happens - ever. However, Blue Mage still learns abilities as one may think they would.
I imagine Blue Mage getting an ability list that looks a bit like the Monster Kill Log that we have (kill X of these, at this place get Y EXP), except imo the bar shouldn't be kills but Blue Aether or something like it. Where witnessing an ability will give you experience to that skill and after filling up that skill's bar you learn it (less random now), and since I envision the Eye as one of the stance mechanics for Blue Mage I think it would be cool if you use this Eye ability and it increasing your Blue Aether receptibility (empowers blue magic/increases blue experience). That way you actually get a bit more interplay than "HIT ME, COME ON AND HIT ME" lol, its more like did you see it? Here is some EXP, did you pay attention to it (via timing Azure Eye) then here is even more EXP (also it wont be a useless skill at cap since it would enhance your abilities too), Immortal Lion stance (for tanking) could be for enhancing the exp learning on single target non-dodgeable spells but I'd rather not encourage players to sit inside an AoE to learn things haha.