Its one of those ironic twists that a game titled "Final" Fantasy has the most sequels of any game save for a few others. Some people say they don't count as sequels, but they do in fact share the same name with a new decimal. That's close enough.
A Realm Reborn - Final (we mean it this time!) Fantasy (same name as one that was clearly not final) just seems contradictory. This game is doubly not final because when it didn't do well, they put in a lot of great effort and remade it. Maybe they need a new related title for their online games.
Thirdly, I know there is lots of value for a corporation in an established name, but these names are getting as long as the names of Mitsubishi models. ARRFFXIVO!
How about Fantasy Online II.