I seriously doubt they will take the Lego brand and just use it for it's "skin" value and have a generic mmorpg underneath.
No, I envision you rolling your toon and building things with Lego blocks especially through manipulating said blocks with the use of your fingers(just like real logos!) Then connecting your creation to other creations(even worlds) and basically the content builds itself with other toons visiting other toon's creations exploring/fighting in their haunted mansion or intergalactic space station.... gonna be awesome.