Elizabeth Lane Lawley has some feature requests that would improve MovableType. Mainly the requests focus on simplifying MT for newbies, which is perfectly fine. I'll take this moment though to document the one feature request that would take MT to the next level for me as a backend hacker: arbitrary metadata on posts.
Attaching key/value pairs of strings to posts can probably be done just by extending the MT::Entry object but getting it baked throughout the entire system involves more time than I have. Still, such a mechanism generalizes a number of MT features, e.g. categories.