How about the following:

If a character is inactive for 60 consecutive days (or maybe 90), they would be granted a one-time use world change ticket. That way a player who takes time off, or otherwise leaves the game, can always come back and join up with new friends.

(I think this would be long enough to eliminate abuse by RMT - and also if a pattern was seen (i.e. on for a day, off for 90, on for a day, etc) it should be easily identifiable as abuse by RMT)

EDIT: credit to Griss for inspiration