[Kiro]

You will need things like client-side prediction, input queues, dead-reckoning, entity interpolation, lag compensation etc etc which means your net code will be need to be tailored for your game. Just outsourcing it like Firebase has done with regular web apps will be really hard without making a lot of assumptions about the client. Adding server authority adds another layer of complexity where you basically need to either run the same engine on the server or simulate the same processes.

With that said, I also want something like this and definitely think it's a problem worth pursuing. I know there's Photon for Unity but I don't know what it actually provides and it's obviously tied to a specific engine.

[aerovistae]

Yes! It is a complicated problem. And it just leaves me laying awake wondering how a 19yo solved it so effectively by himself. (agar.io's author) How could he possibly have such depth of skill and knowledge at such an age?

I know, I'm side tracking from productive conversation, sorry for that.

[halftheopposite]

If you're looking for resources on learning the basic principles of multiplayer games I invite you to read the very well written and illustrated tutorials by Gabriel Gambetta https://www.gabrielgambetta.com/client-server-game-architect....