I was looking for alternatives of MC that are atleast usable, and found a thing called Minetest. This apparently is a Voxel Engine/Scriptable game, where you create games, that consist of mods/modules and other resources.
The cool part of it is, that mods and games etc. get handled by the game itself in a package manager type fashion, so the only thing you as a user have to do is selecting them in the ui, and putting them into your world.
It's this easy because the content is managed by a content database. This engine is built with multiplayer support by default.

Now comes the interesting part: apparently a few devs sat together and made a whole MC clone in this engine, and have called it Mineclone 2. I was testing it recently on a server and have to say, that it doesn't appear to be some low effort clone, but to my surprise is an actual playable and nicely looking game. So far i'm having fun with playing and even modding it.

Since the core is written in C++ and the mods and games content is written in LUA, you can easily writte new stuff for it, and even look at other mods stuff, to find out how to make it compatible or how to do certain things. The licenses usually allows to reuse and redistribute.

If you're looking for something like that, give Minetest + Mineclone2 a spin.