I have friends who are on multiple servers, some in my data centre, and some who are not. This was due to the early problems on server capacity, but with careful timing, we've found ways to play together with some effort and timing using the Duty Finder.

Would it be possible to create an area where people looking to run a dungeon could gather from the entire data centre and party up to run dungeons? You'd, of course, set it up so trading would be impossible inside the area, but you could change your job as needed and then activate the duty finder which would fill in any empty spaces in your group as it currently does.

I believe this could improve the game play experience for friends who share a data centre, but not a server. It could be either an area/zone accessible from each server, but you can only return to your own, or it could be a place you can choose to log into from the character selection screen instead of your server if your intention is to run duties with friends.