There was a small rumour a few months ago about LittleBigPlanet 3 being made, but not by Media Molecule. Sumo Digital was the developer listed.
I would love to see Media Molecule make a LittleBigPlanet 3D. I would love to see the results if they could design the tools needed for people to make 3D worlds.
Alas, they're busy with Tearaway and their PS4 project (which will no doubt be awesome).