Cross Data Center will never be a thing, it defeats the purpose of having servers in different regions, and puts load on services that simply can't handle "all players" logging in at the same time.
There still is a potential work-around that I'm surprised SE has not proposed. Allow a "echo clone" mode, that copies the state of your existing character, minus inventory (eg only gear currently equipped will be taken with you) to be able to temporarily exist on other servers/dc's:
Limitations:
- No loot rewards
- No money
- No gear changes, no repairs unless a player can repair it for you for free
- No shops
- No trading
- No Chocobo, no mounts (other than zone-specific mounts as MSQ rewards that won't be rewarded from a clear)
- No Link shells
- No FC access
Basically you're there to fill a slot in a party, or for RP reasons. Once you disconnect from the world, your temporary echo clone is deleted. This would of course not be permitted at all if the world has any queue. That circumvents the issue with having to synchronize any data between the DC's, it's just a one-way trip, and you can't bring anything to the world, or take anything from it. If you want the rewards, you have to have an actual character in that DC.
There's probably more data points I've missed, but the general problem with cross DC is that it would require synchronizing both DC's server clusters, not just the servers. With CRZ's (Cross Realm Zones), this is actually internal data sync, and if they're really planning on making the DC's one "super server", then it's very likely that they've decided to break up services to their own servers/virtual-servers inside the DC so that the only thing that a "world" really represents is a set of non-overlapping zones for capacity reasons. They could easily go "ok this new area, we're going to have five instances of it (one per world) only", and players are just bumped to other world's versions of that zone if the zone is full or have explicitly selected it when partied with players on other servers. Areas that are generally empty, would just collapse into one zone.
So I just don't see this happening if they physically move the DC's away from each other, as that would put a prohibitive cost on synchronization, where as if it's physically in the same DC, it costs them effectively nothing.