This could be interesting. I feel like the problem with MMOs that give you too much freedom is how players with more time will just completely dominate everyone else within days of any new content launching. Also, in my experience bad/unfun behavior in general gets worse the more populated an MMO is (FFXIV being a nice exception), and this solution could help keep traffic down. The only problem is that no dev trying to make money would ever time-limit their players.
Perhaps time limited but only per realm/server/world? That way someone trying to get their fix can play across multiple isolated economies but still allow players to play more if they really want to (lets be real people would multiaccount anyway)
This was usually done by having separate instances with different time limits. That way all the lifers with 12 hours a day to spare could play together and let everyone else enjoy themselves.
