It would be nice, and we did in fact have an NPC companion of sorts back in 1.0, but then they replaced it with the chocobo only to then abandon said chocobo.

With the addition of BST and its gimmick being companions though (albeit monsters), I don't know if they'll give us NPCs in the over world for other jobs to use (and if they do, they would likely just default to the Scions instead of any kind of custom NPC).