Is it possible for FFXI & FFXIV to merge?

[Kurogaea]

No. Lore wise it is not feasible.

The two are connected with the Void, but they're different worlds with different mythologies and different rules.

As if gods couldn't consult with one another and the fight between good and evil couldn't overcome boundaries of the universe. If Shantotto can break the barriers of worlds and come to take over our world (and we kick her butt) then whats to say the worlds couldn't coincide with rules and the like. Also who's saying that the worlds would be completely bound together? We could just open a rift for the warrior of light and a story driven npc who happened to find or unlock the route to this new world. Lore is hardly a problem, like ever. Allagans are still a story convenience. :/

[Catwho]

As if gods couldn't consult with one another and the fight between good and evil couldn't overcome boundaries of the universe. If Shantotto can break the barriers of worlds and come to take over our world (and we kick her butt) then whats to say the worlds couldn't coincide with rules and the like. Also who's saying that the worlds would be completely bound together? We could just open a rift for the warrior of light and a story driven npc who happened to find or unlock the route to this new world. Lore is hardly a problem, like ever. Allagans are still a story convenience. :/

That's just it though - SE has a name for Shantotto and Lightning (and Gilgamesh) - and everyone in Dissidia. They are Voidwalkers and their ability to travel between universes is canonical in their originating games. (Shantotto had a whole mini expansion dedicated to her misadventure with clones in the void.) FFXI also had its own parallel universe mini expansion set with Abyssea; Eorzea is not the parallel world to Vana'diel, Abyssea is.

But to go along with your argument... could they? Sure, they can shoehorn anything in and blame it on the Allagans.

Should they? No. Absolutely not. I stand by my answer. I played FFXI for ten years. Let my Tarutaru enjoy her retirement in the Mog Gardens of Adoulin already.