AOSP is mostly licensed Apache and LGPL. The repos have probably been cloned a million times. Although Google has taken large swathes of the project (like the launcher or Settings app) behind closed proprietary doors. But that is not a huge obstacle, there are already plenty of decent open source launchers.
The most important thing would be a (drop-in) alternative for Firebase Cloud Messaging. Without it, you can say bye bye to any decent battery life.