I'd build an MMORPG, with heavy emphasis on RPG. Open PvP, but griefing is completely intolerable. If you start running around and killing everyone just for fun, you're banned. If you grossly break immersion (out of place character names, excessive OOC chatter in non-OOC channels, et cetera), you're given a verbal warning, suspended, then banned as incremental steps. The GM staff would be on hand to help with bugs and complaints and the like, but also to run big and small events all the time. Basically, I'd recreate the wonder of EverQuest. Back when the world was huge, GM events could happen at any time, and the only people playing MMOs were the people who wanted a mix of Dungeons & Dragons and LARP. It wouldn't be profitable, and I don't care. For the people who actually enjoy an entire world to ROLEPLAY in, it would offer something that no MMO dares to do.