What would be really sweet is if the list of next methods that are passed to method functions is actually a list of method metaobjects, not method functions. This is how it is specified (but I'm not testing for this in MOP Feature Tests). It doesn't seem that anybody uses this feature, because this is the place where the various MOP implementations vary the most.
However, on top of that it would also be good if call-next-method would not rely on a special variable holding the next methods anymore, but just refer locally to the next methods passed to the method function. I don't know how ECL handles tail calls, but if it performs tail call optimizations, then it would also be able to do this for methods (which is currently not the case due to the reliance on a special variable here).