The easiest way to pull off your own MMORPG is to buy the Old Neverwinter Nights Gold or Platinum edition and start from there.
I modded several years for both NWN and NWN2 and still play on community made servers which go well over a 200(!)
Some people who retired from running their own NWN servers even put up their whole world for everybody to download like this one:
http://nwvault.ign.com/View.php?view=Modules.Detail&id=5739
So you can take a whole functional MMORPG and tailor it to your own style.
However 3 things have to be known:
1. All NWN community content is still the IP of Bioware so no games/servers may be commercial.
2. The old NWN engine (Aurora Toolset) is darn easy to learn but you still need a team AND tons of time to actually get your own server running.
3.Standard single servers can only hold 64 people (but only the most popular permanent worlds are full at peak times.)
In case you find my grammar and spelling weird ---> native German speaker ^^