LITTLE BIG PLANET AND TERRERIA!
actually that doesn't sound half bad.
Anyways, I've thought of this myself. Since I'm sure the product game from this idea would probably suck major ass for some, it'd probably be a good idea to remain in dev for both of the original games as well.
In other words, it's kind of a waste of time. I don't think many people would like whatever this would be, although it may not be half bad. However, the two games are built on seperate engines, Blockland being Torque and ROBLOX being a personally-built one, IIRC. That, as is, is probably enough to say that this is simply too much to really put into rational thought.