Cross-Datacenter would likely have horribly impacted performance given the latency required to sync things up. Players for Global FFXIV (so not China and not Korea) can already make characters on any of the datacenters and play on them. The latency between a single player out of region isn't too bad. If you had to introduce player <-> local world <-> local datacenter <-> remote data center <-> remote world <-> other players, you'd be adding on a much more significant amount of processing and latency.