I'm not so big fan of videogames, and i don't play or complete alot of it, and it's my opinion. I just see in it the perfect, because it's ultimate and extansible game.
Unlike Minecraft, you have a modding API out-of-box, so you can just write it on Lua, and it's works fast.
GNU loves Minetest, and give it to you as alternative to Minecraft. Also, not only game opensource and free, it's mod's and textures.
I have a lot of memories, when i play old Minecraft with my brothers. It's being a great time, we will don't have so much blocks, we play versions like 1.5.2, and MCPE 0.9.0, it's being time, when we think, how we can make as cool, as possible buildings, and it's look and feel really comfy. But now, we will grow up, first is in Katowice, and other in Odesa. So, i have something samailar in minds with brother from Poland, so maybe we can recover this times.
You have a lot of bimes and map generators, some of them looks really interesting, like "fractal".
Unlike Minecraft, you can use shaders out-of-box, and it's looks cool.
When you go to "Settings" > "All Settings", you see a lot of options for basic games, and it's mods. I never see something more customizable, and it's give you possability to have interest to this engine almost forever.
On Minetest's forum you can found software, to convert textures or maps from Minecraft, so you can easily leave Minecraft, if you want this. Also, some peoples trying to recreate mods and textures from Minecraft as it's being native.
Unlike Minecraft, you don't need to install something, to run your world on server, you just can do this in-game.
You can found a lot of servers with official server list in-game. Someone can said, that is have some issues with security, and maybe, you are right, but just play it with you friends on own server.