Second life is an mmorpg that is based entirely on player-generated content. The modelling and scripting tools are available to players to make material in the game - to the point that, if you wanted to program a system for warfare, you could. The only problem is that you have limited resources - you start out only with virtual money, which is used to design and own equipment, and no land. Land often costs real-world cash, because it represents server space that the administrators must pay for. Therefore, people can make subgames within Second Life, but if it requires a lot of resources then it will require either the cooperation of a large group of users (to amass the virtual money and get some land) or the administrators (if you show programming talent and drive they''ll often throw in the resources for free) or some raw capital.

Still, its currently the fastest way to get into the "I want to make MMO stuff!" world.

I''ve never played WWII Online, but after playing Planetside ever so briefly I''m fairly convinced that a MMOFPS game has yet to be within reach. FPS games typically need alot more bandwidth than a hack and slash RPG, and it is in this department that alot of sacrifices were made in planetside which IMO totally removed any feeling of FPS it had. It felt nothing like the faster paced action packed FPS games I love to play. Point is, you have your work cut out for you if you''re doing a MMOFPS, which is what your idea sounds like, even though you specified RPG. Sounds more like an FPS with role playing elements and persistant characters.