First a question to your question: If you change the rules so much that it is no longer d20, does it really matter?
Just curious on that one. d20 by defintion is anything that uses the SRD. OGL games by defintion move away from that, so I think tecnically going classless would by OGL. But I don't think that's really what you are talking about.
d20 is just a mechanic. Roll the d20 and add some things to get a result. So, maybe just having that as the primary basic rule is all you need.