It would be a hard fork which Google is forcing anyway. That’s the path Graphene is taking.

It’s start with a basic working phone gui and apps and improve it (Graphene’s path) or start with absolutely nothing and build everything again (Pine OS etc’s path).

I don’t see why you’d need to run pure Linux over Android. Android phones at their core are a Java app running under Linux. There were even lawsuits because Android was originally a Java app.

Working on the open Android projects seems more productive.