I still would love to have a Blue Mage as a potential main, perhaps the nostalgia speaking strictly here but I loved Blue Mage from FFXI and was really hoping/looking forward to giving it a FFXIV whirl.
Said it before but while I would agree that making Blue Mage a main job would absolutely take resources I would add it's probably one of the most cost effective decisions they could make (on adding a "new job"), so it's resources but likely not as much as usual. Since they already have many concepts, beginning lore/introduction, base localization, basic systems, ability icons/spell visuals and effects, animations, and more implemented into the game. I don't want to say that means it's easy per say but that I do think Blue Mage is an "easier option" and maybe might make something they had originally said "never again" with Ninja a possibility again.
That said, while I would be a bit sad always thinking "what if" I could main Blue Mage, I could live and enjoy blue mage being truly limited if it just (imo) felt more to "can't be balanced", felt more ridiculous, as well as mechanically smoother (there are many cast spells, few spell interactions, and no combos). Making it just a monster of monsters. Imo, if we're going the "can't be balanced route" I'd like to see Blue Mage be able to shrug off "those can't be done solo" concepts.
"Titan casts Gaol"
"Blue Mage landslides Gaol over the edge" (before the Goal's were changed technically blue mage could freeze them lol)
"Nael Deus Darnus casts way too big meteor golem with super regen hp"
"Blue Mage removes all the golem buffs leaving golem stripped bare and kill-able"
"Boss casts sleep"
"Blue Mage has Acorn Bomb equipped which gives a passive 100% sleep resist"
"Stacking debuff that's unavoidable and requires split to survive is applied"
"Blue Mage removes debuff, because no"
"Goobbue goes to draw in Blue Mage for instant kill mechanic"
"Blue Mage casts spell to shed skin leaving the mechanic target on the molting remains as their true form breaks away"
I think to me if Blue Mage was a bit like overpowered trading card game with more interlinked mechanics I'd have fun with it as that. But currently, to me, it feels weak for what I already have (levels would help obviously), and particularly weak when considering the high grind, and with not as much flow as I'd like for a job (rather than playing cards that play off each other a lot of times I just feel like playing unrelated cards down, and occasionally a truck load of buffs just for one spell). I get that each job has a different style and not all styles each person will like (so maybe that's that), but I feel Blue Mage is missing some of that glue between skills since there are few interactions, easily resistible or already 100% resisted buff based spells, no combos, and not really a system to encourage diversity aside from going out to learn some oGCD + buff your big bursts.
Like I said I think a main-able blue would be cool (at least coming from a "I loved FFXI blue" perspective), but I would have fun with a job that's basically a trading card game where you have multiple sets of "decks" because each one is uniquely intertwined and powerful. Like a Diablo 3 legendary affix combo system where one item lets your Barbarian jump three times, another lets you cast avalanche on each jump, another gives you energy at each avalanche cast, another makes a high burst spell no cooldown but have cost, and all a sudden you've got a set of things that come together into a hurricane of madness. Yet you still go and try other combinations because it's just fun to fiddle with the affixs. I'd love to have that feeling when SE adds a new "card" to blue mage that my "deck" is shattered in possibilities (I get that couldn't happen all the time, but that sort of feeling when new cards are added to other TCGs for example).
Aside from that I still think perhaps a failed spell learning buff could help quite a bit, each time you fail to learn a spell a buff is added (until log out or successful learning) that increases the chance. At least for dungeon/trial spells.
Also I thought the idea someone added that spells add to Blue Mage's ilvl could be a really neat idea. Since it means every spell is valuable even if you have no intention of using it and the Blue Mage isn't necessarily OP out the gate but the more you collected the more you'd just get a passive buff to your character.
Hopefully this 5.1 new content will do something to spice stuff up, iirc Yoshida said there would be new spell mechanic or something on top of new spells. Not sure if that means some of the new spells would work together like Dragon/Ram Voice, if they added a sort of "glue" to make Blue Mage spells have a tighter connection, or what have you, but I am looking forward to what 5.1 brings and if perhaps any feedback was considered.