Anyone can make an open source game like this. I'd rather see a successful one.
Besides, what possibilities? I'm a huge proponent of open source but there's more or less nothing else that you would be able to do if it were. In an MMO, you abide by the rules of the central server, no matter how open its source is.
If the server software was opensource you could host one yourself and decide the rules, make custom game modes (capture the flag for example) and make the game work the exact way you'd like. For example, I could imagine this being very fun for disabled people in wheelchairs, but the default gameplay should probably be changed.
Besides, what possibilities? I'm a huge proponent of open source but there's more or less nothing else that you would be able to do if it were. In an MMO, you abide by the rules of the central server, no matter how open its source is.