Given that they've all but eliminated dedicated debuffs(save sleep and debuff effects on certain spells), and given that Mystic is already in the .dats, I wouldn't be surprised if that ends up being the defacto debuff caster in the future, although it's up in the air whether it'd be a class or a job. If it's the former, then I could see it leading up to Time Mage potentially, which would be pretty epic. Yoshida has mentioned before(half-jokingly) that when(or if) they add Arcanist, it'll be trap-based and probably lead into Geomancer.

Given that Archer leads up to Bard, it would no longer surprise me if they had Fencer(assuming they add it) lead into Red Mage.