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.

I was thinking of something similar recently as I am a big fanatic of PvP games of different kinds. Problem is as I get older I have less time to play to keep up with my enemies and would love to have "adult" servers which are only on at certain times of the day (maybe even with some auto grinding on the off hours). To allow people to be on more even footing. I bet there would be a decent chunk of people who would enjoy this.

Turn based games can work well.

There is a multi-player browser-based version of Mike Singleton's Lords of Midnight that takes everything great about the original and pitted you against real opponents.