*cracks knuckles* Oh boy, bringing up my favorite job.

To be honest, BLU could very easily fill the niche of a melee caster. And, while it may seem lame to fans of blue magic, the monster hunting to learn their skills could easily be wrapped into the job quests.

Quite frankly, Blue Mage could be easier to implement. Plus sword DPS.



Buuuuuut~
GEO, on the other hand, has varied quite a bit in appearance and skills, and could easily be modified into any role without breaking classic lore.

With that said, Terrain as it's usually used simply couldn't work in FFXIV as it could put parties in a position of heavily favoring or excluding GEOs based on the effects of Terrain in different instances. They'd be far too situational for the trinity system.

.