If they're able to pull it off then it would be cool, but the only non-SE crossover we've had was Yokai Watch and that was only because friends with the producer of the game. Honestly I think the chances are pretty slim that we'll get another crossover with a non-SE game, especially since there were some players that hated it because the event "broke their immersion."

If we do get a crossover though it would be pretty weird and it probably wouldn't be anything deeper than the Lightning Returns event where Haseo finds himself magically inserted into the world of FFXIV and gameplay with him would be tied to FATEs where you would fight bugged enemies. Rewards would also probably be just Weapon Skins (Epitaph Weapon Skins, XD) along with more iconic skins such as the Chainsaw Sword and Haseo's starting daggers.