I have to admit, I am quite surprised by this. Mojang (owned by Microsoft) are to open source more of Minecraft and they've already started to do so.
Beginning with "Brigadier" [GitHub], which is what they're calling a "command parser & dispatcher". What this does, is deal with commands players can enter into the chat box like "/give LiamD gold_block 4". Then, it splits it up and parses into ways the Minecraft actually understands and hopefully you get your gold blocks. Note: I posted about this on our Reddit nearly two weeks ago.
While that's pretty cool, it's not massively exciting overall. However, this is only the beginning. They've also open sourced "DataFixerUpper" [GitHub], which deals with old and stale data which needs to be spruced up when loaded into newer versions of Minecraft. The example given is from older Minecraft worlds, where perhaps a part of the world hasn't been loaded for years and this helps deal with those situations.
The most exciting thing though, is that they're considering open sourcing what they're calling "Blaze3D", which is a brand new rendering engine they're hoping to put into the 1.14 release of Minecraft. Now that would be pretty awesome to see.
See more about their plans on the official Minecraft blog post.