Doesn't Rocket League currently have crossplay support for PC and PS4? If Psyonix wants to maintain that model, X1 is an absolute no go. The big issue with getting FFXIV on the X1 was the fact that Microsoft is stingy as hell about allowing crossplay. They demand exclusivity on some level if a developer wants to work with them which is a huge barrier in an indie multiplayer focused crossplay release.
And no, it would not be easy just to do cross X1 and PC since that would mean making all kinds of exceptions about who could connect to who that nobody would want to bother with.
Microsoft has gotten a bit more indie friendly, sure, but there are still massive barriers that should be fairly obvious considering so many indies go from PC to Playstation first.
