It's doable, FiveM have accomplished it, even. I used to host dedicated game servers back in the day when that was a thing you did. Most servers had custom var

It's doable, FiveM have accomplished it, even. I used to host dedicated game servers back in the day when that was a thing you did. Most servers had custom variables set for all sorts of stuff: health, gravity, respawn time, etc. All of these sorts of settings exist in GTA, and Cyberpunk, and Halo, and so on. It's up to the developers to expose them. It seems perfectly reasonable to me, and likely you, that Rockstar could charge to host a private server, provide admins with all the knobs and levers they could dream of, and even provide access to scripting and resource primitives that modders will figure out how to access anyways. But they probably won't. If they were really smart, they'd just buy FiveM, and have GTA6 ship with paid modding support.