Now I know for some of you "Kickstarter" has become a bad word, but you just might want to take a look at this project: Voxelnauts MMO
It's a sandbox voxel-based MMO in the tradition of Minecraft, but is far from a Minecraft clone.
One of the many things that sets Voxelnauts apart is the developers' decision create a "metaverse" of user-created planets.
They're developing a system in which users can basically program their own planet to be anything they want it to be.
Using LLVM JIT (Low-Level Virtual Machine Just-In-Time compiler) They've made each game element basically its own unique program compiled on the fly.
Think of it like an operating system running multiple applications. As far as I understand, the applications (game elements) can communicate with each other, but their code is independent.
This introduces great freedom and flexibility into what users will be able to create, something we haven't seen before in an MMO.
The graphics are voxel-based, as the name suggests, but they're using sparse octree ray tracing which gives great performance, and a truly beautiful experience.
I mentioned it the title of this thread, but the game is being developed on Linux, so there will be support for that platform guaranteed along with Windows and Mac.
The team is also ensuring the game not only works with VR, but is designed for it, although desktop users will still have a perfectly normal experience.
There's a lot more to say about this game, but really, you should just go check out their Kickstarter page for yourself.
As a beginning hobbiest game developer myself, I'm excited at the opportunity Voxelnauts could give me to learn some things without having to start from the ground up.
I'd encourage you to give it a thorough look, even if you are wary of Kickstarters.