I would love to setup a master server browser list that updated in real time so that you can see by the moment the availability of servers and stats of current players in the server. I think Firebase would really make this electrifying! Right now the delay that Steam's master server browser list has is just so 1998.
After that I'd add realtime global chat to the main menu of the game and server administration without having to log into a server. Then maybe I'd play around with doing some funky stuff where I push player locations in real time to Firebase and use that to drive a webclient that just renders the map artwork and player location dots.
So many possibilities (!!!) but without a true C++ Firebase SDK I can't justify the time to develop these features or the SDK myself as I have a boatload of other tasks to complete for the game.
If anyone is interested in freelance C++ work and thinks they could create a true socketed Firebase SDK solution (not REST) in UE4 send me an email at contact@drivenarts.com