Would be a bonus if you could hop between Vana'diel and Eorzea, and it would mean you'd be able to continue Vana'diel's storyline. Probably a significant cost issue to do so but I can see adavantages to doing that (Plus a unified sub fee and no more POL)
Funny you should mention that because this idea was floated very, very early on in FFXIV"s development, around the time they revealed that FFXIV would have similar races such that people could recreate their avatars if they wanted to. The idea was there'd be a portal in your mog house that would take you to FFXIV's world, which would load up the other game, and you could go back and forth. This idea was scrapped pretty quickly however as unfeasible.