Recreating PS presents a lot of challenges. I've been reading about the method that TD likely used to make the k-code system and to encode the UGC. It sounds like a lot of work. It sounds easy to infest with bugs(which we experienced). To get it to work REALLY well, the complexity ramps up significantly, which means way more coding time on the back end. And that's just to get the tiles to compile into something. Then you have to do an entire 3D environment engine, then you need to license or code a physics engine, make a framework to hold it all, and all the networking coding to pass everything back and forth. And that's just the coding. Then there's all the artwork, all the modeling, all the animation, all the rigging. There's a reason it took 20-30 of them 3 years to produce what they did. Might be a fun thing to work on if you didn't might releasing it in 10-20 years.
Number one problem with getting to MS to open source Project Spark is that the physics engine was licensed, so basically they can't since the physics engine is integral to the game engine. From my understanding they also used proprietary tech for integrating their model, rigging, and animation in some fashion. They likely would not open source that.
That said, Kodu Game Lab, upon which PS is partially based IS open source, so if you wanted to do basically what TD did, you're technically free to do so. That would probably eliminate a lot of work in coding the k-code system, but you'd still be missing ALL of the art assets, and all of the refinement in other parts of the program, which is not something that could be recreated overnight.
End of line.