I'd be interested in a backend service that does auth, store inventory of purchased in-game assets, etc. Something simple that's easy and fast to standup.
What kind of game is it? Requirements for a MMO will be different to say, a shooter, or something with a leaderboard.<p>Firebase is usually the default. It's not as simple as it looks and can be a little costly, but it's well supported.<p>If it's just CRUD, Parse Server is much faster to set up and cheaper. It's was open sourced by FB and suddenly everyone lost interest as it was no longer a resume item.<p>If you want something more customized, less simple, but with a little latency, serverless stuff is fine.