Waiting for 6.4 was the original plan. However, every new diplomacy feature request/promise suggests 6.4 won't be released any time soon.
I don't think 6.3 is stable enough for multiplayer, so if we got impatient and started now we would probably use 6.21.
In my opinion, the 6 biggest multiplayer-related improvements I think we would loose by starting now are:
- Ship/commander kill histories.
- Correct alien class names
- Shock Damage
- Fighter Bonus bug fix
- Interception movement
- NPR update package