We're always looking for people to contribute to our project. We make heavy use of C# at the moment as this is our primary tooling language. Have a look at our recruitment post and find out if this is something you'd like to participate to.

https://gamedev.net/forums/topic/711264-looking-for-additio…

Regarding MongoDB, there is also this: http://jepsen.io/analyses/mongodb-4.2.6​ showing that the MongoDB transactions don't actually work in case of network failures.

If you really want to go key/value or document store, the good news is that FoundationDB has been open sourced by Apple. It used to b…