I actually wanted to build our own server for many of the features that Gamesparks offers but just realized that I can probably save myself a *lot* of time ;-)
Our game currently uses Steamworks API heavily and will probably continue to do so in addition to Gamesparks for players that actually use Steam. However, we also plan to release for other platforms, like Oculus Home, Viveport and PlayStation 4, and would like to have a unified experience in addition to what the individual platforms offer. Also, some of the things we want to do simply are not possible using Steamworks API.
A few words about the game: Holodance is a Virtual Reality Multiplayer Rhythm game. After about a year of Early Access, we have sold about 4000 units, and given the adoption of high-end VR (HTC Vive, Oculus Rift + Touch controllers, PlayStation VR), if we ever get to 100,000 units sold, we'll call this game a huge success. So even 1,000 MAU would be pretty big for us.
So here is a list of requirements we have (a few are already "solved"):