This would be a very mammoth task indeed. Even having XI's current assets the Development Team would have to re-create almost all of the assets, be it models, monsters, areas, as; due to their age up-scaling wouldn't be efficient nor look pretty. While it would be nice to perhaps see Vana'diel (perhaps void of life and a barren wasteland?) the task of combining the two Games isn't something that's easy, and would take many months perhaps even a year or so to recreate everything that XI has to offer.
Perhaps you should look forward to what Yoshi P has in store for us, and let the game shape itself and take on it's own form. ^~^ It'll take time, but I know Yoshi P can pull it off.