I like the idea. Actually I wanted to write such patch myself, but this is very delicate thing and needs to be very careful. The largest benefit is not just memory saving and performance, but robustness. Currently it is easy to went OrderedDict in incorrect state by using pure dict API. This can cause crashes, hangs or invalid bahavior (see issue24726 and issue25410). The new implementation should pass all existing tests and also fix the above issues.

See also issue28239. lru_cache uses simplified version of ordered dict.