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.

People have done it. The problem is the huge binary blobs for hardware. Its difficult without the hardware company themselves giving you the specs to make your own.

One of the reasons fairphone is supported on many alternate platforms.