To be honest I'd be really disappointed if Blue Mage was tied to the Garlean magitek lore. I'd much rather see Gunblades used to make a ranged or dex-melee DPS. Either is good imo. This is mostly because it's a way to give a nod to the later FF games that had magic/melee hybrid dps characters, including a few that used Gunblades like Squall/Lightning. I doubt they'd reference those two too heavily but their Dissidia iterations have a lot of inspiration to draw from.

That's not to say that I wouldn't enjoy seeing a Blue Mage, but I think it's more likely to come from one of the provinces in the way to Garlemald, rather than someone from Garlemald itself, perhaps they wind up in Doma as a refugee due to the civil unrest in their area, which we're likely to be trailblazing through in the next expansion if we follow Thancred.

I'm for it being a tank, mostly because I suspect it'll use a resource that relies on getting hit to access its blue magic if it is made into a tank, and like many have suggested, learn its abilities from quests rather than the overworld. I actually think it may borrow from some concepts in SMN and WAR's respective lores, specifically tapping into the aether that was washed over them by the primals and activating it in order to enhance themselves at the cost of subjecting themselves to some of the influence of the primals. Basically it would be a soft-tempered/drowned/etc state that preserves the caster's mind but gives them the ability to learn monster abilities and transform temporarily in order to use them. I can see it as a Melee DPS or a Caster DPS, but healer is highly unlikely. Given its healing abilities are limited but applicable as utility I suspect it'll fit better as a tank or Caster than as a melee.

In all likelihood it'll use Maces (2H or 1H, possibly dual wielded) regardless of its eventual role, unless there's another class planned to use those down the line. And Heavy armor fits if the intent is to get hit. It'll likely share armor with Dragoon if it's a melee dps for that reason too.