The option I'm batting for, merge all 3 servers. This will Solve population issues at least until paid transfer service (Or whatever is planned later down the track) where people whom are asking for transfers to American servers can do so. (I'm obviously not 100% sure, but I feel like they would be the minority of people who want SOMETHING done.) I also think that merging the servers into a PVE server and people experiencing the larger population that all 3 would bring may reduce the number of people wanting transfer to American server (Obviously you'll never really satisfy everyone though.) I do think that keeping an APAC server open will entice future APAC players to join the game and I know many players who will leave the game if the APAC servers are closed.
Edit: I should also add I think opening up transfers to just anywhere before this issue is resolved will not fix the population issues but only serve to make the population issues even worse than they are.
This. Merge to a super server.
When transfers become available as a paid norm then those that wish to can.