I think ASDF should not change the semantics and break existing system.
We can provide new, improved functionality without breaking anything. And keep old systems working forever with new ASDF.
Lets fist consider and decide, how exactly the best support for syntax customizations should work.
Withoug compromises (like allow shared readtable modifications, but resetting *read...* variables - that's strange compromise). Lets ignore backward compatibility for now.
Best regards, - Anton