I know nothing about mobile OS development. How realistic is it at this point for someone to fork Android?

I know that software like Edge and Iron Fox can be forked and worked on by independent people. But Android is developed to integrate tightly with proprietary hardware.

Forking the OS isn’t a big issue. The big issue is people installing the fork.

To install custom rooms you have to unlock the bootloader. Which can be easy, to impossible, depending on the phone.

But unlocking the bootloader breaks many of the security features. Stuff like NFC payments no longer work. So basically the phone is crippled.