MMO ideas are like (insert your own analogy here) - everybody's got one, and they all... whatever. But anyway, here's mine. Just so my reader (if I have such a thing) is aware, I don't really play MMOs. World of Warcraft is not my thing. There are a few MMOs coming out that look promising, but as of right now, I don't play.
So far I'm calling my idea "Sovereign." Honestly, it wouldn't matter whether it was sci-fi or fantasy. I would like to see a game that encourages people to really group together. None of this "Yeah, well, so-and-so is in my guild, so you don't want to mess with me!" garbage. I mean get together and make a city.
The player would start in an existing city where there is no PvP. There would be NPCs that could do pretty much anything you need - sell, trade, train, whatever. This stuff is pretty normal.
In that city your home would be in a sort of apartment complex. You might give the player an instance of their own sort of home that they could play with a bit, kind of like Playstation Home has now.
What would set this apart though would be the ability to leave the city, go to the PvP area, and build a home. This would be a persistent addition to the game, meaning that even after you log off, the house is still there. Here's where the problem with a persistent world becomes the strength of this game. So, you're not online. Your house is just sitting there, and you're hoping no one attacks it. What's the best plan? Some security would help, and that would definitely be in the game, but the best way is to find someone who can defend it. And it would take more than one guy to have it always defended, so you'd end up with this small community of people all helping each other out.
It would of course be best for that community if each member had something more to contribute than just a sword (or whatever weapon they have). The community should be self-sustaining. Forget the NPC's. Let the players have professions that help each other. Soon those communities would become cities and maybe even countries. Alliances would be formed, wars would start. I'm sure you see where I'm going with this.
I would also allow for players to become some sort of gypsy, nomad, mercenary, traveling salesman, or other type of character that doesn't necessarily have a home. Maybe a vehicle for a home. Not everyone feels like being friendly.
Okay, so now the weakness in my plan. It would only work if there were a LOT of people in the game. No, we don't need 11 million like WoW, but there would have to be quite a few, and it would probably have to be worldwide so someone can watch your house at night.
The technical requirements might be a bit of a pain, but not impossible. We already have games where we can build our own stuff, and we already have vehicles. It would also require a lot of professions. A bunch of mini-games would have to be thrown in for those professions, like raising livestock, mining, that sort of thing.
Anyway, the benefits of this kind of game would mostly be the sense of community that would develop. The other would be that players would have a sense of obligation to continue playing on a regular basis, because other players depend on them.
So there's the overall plan for an MMO. I have plenty of ideas for some of the little details, but they're things that I'd like to see in any MMO, so I'll save them for another post.