How do ascians come into existence?

[Cilia]

It is, but on the other hand: We already had more than one visitor from Vana'diel, namely Shattoto and Iroha. In that sense, they've already gone back on that, as a cross-over requires some form of connection.

In general, there is a faint connection between all Final Fantasy worlds through the Interdimensional Rift. This is how Gilgamesh appears time and again and, unlike most recurring elements of the franchise, is the exact same being who appeared in V. More than Shantotto and Iroha have appeared in Eorzea thanks to that - Gilgamesh, Ultros, and Lightning have all made visits.

However, having characters from other games appear in temporary events is quite a different thing to having Hydaelyn linked to the Ivalice. Grafting one setting, or a facsimile thereof, into another sends up some red flags for me. The interdimensional rift angle is even worse.

That's not to say XIV Ivalice is or will be the Ivalice, or a facsimile thereof, or that we will be going to the Ivalice. I'm still holding to the "wait and see" approach. However, with it having been said before there is no link between Ivalice and Hydaelyn, and now Yoshi-P's saying it's not just an homage... alarm bells are ringing.