This is the sort of game that is best made through an accumulation of your work over the years. If you just go 'let's build an MMO!" you are going to have a really hard time, and I dare say never even get close to finishing it.
Break it down into what you need to do. Combat systems and text systems are amongst the hardest things to make, and you need to make sure that your design is 100% compatible with MP. I spent some months on a combat system and earlier this year had to revamp it considerably to fit into MP. Then just this week while working on the New Grounds comp I found a much, much easier way of handling all my customisation menus, which if I were to make a full MMO would save me literally days to weeks of programming.
Also bear in mind that C2 has no inbuilt login/online storage yet, meaning you need to find other ways of going about it. I read somewhere that they plan to add it later though.
So my advice to you: build tiny games, each using a different component of your grand plan. Make a quick text-based adventure, some arena combat game, and some light puzzle adventure for example. Try to polish each of them up to a releasable standard and then you will have a much better idea of what you're in for, and have something to show for your efforts. Which is a very good thing, because honestly, you are not going to make a grand-scale MMO by yourself in less than a few years - if at all.
Oh also, and this is actually the hardest part I have come across, find someone else to program with you! You will see that a lot of people here don't do it simply because of the difficulty in merging your work (SVNs even make it worse in my experience). But when you get a structure together and know how to lay your work out in a way you can easily merge, you are going to get things done sooo much faster and easier.
Anyway, good luck to you!