Easy answer...Summoner. It's not FF without a summoner! Ifrit, Titan, and Leviathan are already coded into the game along with the fact that the LORE says we can summon primal's. Would I play it? I have no idea, but it would go ALOT further then any other class you can add I can think of to both fitting into the game/lore and making it feel more like an actual Final Fantasy title. It's a pretty bad joke it's not included atm and with yoshida stating he doesnt have any plans for extra classes it's a shame. Fix the current ones sure but you can still add others to flesh out the game as a whole, especially considering how many teams Yoshida has working under him :S