Do you really think a JEP is worth investing my time in? AFAICT, the Java gods have already made up their minds on extension methods.
But tbh, there are some very good arguments against extension methods. Also, some tricky questions about overloading handling and compatibility are lurking in the shadow. But if you have good suggestions on how to fix them, I'd take the time to propose something on an OpenJDK mailing list, maybe you get some support.