Dissidia laid a lot of groundwork for crossovers with the story revealing that powerful gods of the FF series are able to create their own dimensions, and even create "clones" of people from other worlds. Also it was revealed they don't have complete control over either of these, since even though the rejected clones were banished to the void, the villains in Dissidia were still able to find a tear in the world, then bring them through (although this was mostly exacerbated by ex-deaths intimate knowledge of the void).
So who's to say we couldn't have crossovers by way of clones from the dissidia universe that are "nearly" complete, but still were banished to the void, finding a way through one of the many portals to the void in the FFXIV world?