could aways do what some F2Ps do and have limited free slots. Also, to stop combat griefers F2Ps could have less damage and less weapons to access etc. Sure it would be limited play but the idea to is nudge them into paying for it in the long run.
I have a similar idea that could be very very cool:
- download client
- don't register (stay anonymous)
- don't login but press a button "play without account"
- play casual style, just to enjoy landscape and see if the client works and stuff
- no chat, no forum, nothing
- but.......
=> but you can't play a human. You get to play a mob (bear, dog, whatever) and will not be able to do anything but attack humans. This will be much like being an anonymous spectator but you will lend your "intelligence" to guide that special mob and make it more life-like for the real players. Get hunted and killed... but you can "get in" casually, anonymously and for free...