Here's my wish-list:
1. A template designed for "non-hosted" composers. Black-edged, with a large hazard symbol to say: DON'T UPLOAD! I would have attempted this myself, except I couldn’t see exactly from where the existing template gets pulled in on the Category pages, and it just doesn’t work on the non-Category pagespace.
2. A new "request additional copyright review flag" that can be raised by general users.
(These were discussed in the moderation forum; I might be able to move those threads to the feature requests forum, if anyone's interested in looking over them.)
3. Enable interwiki links for IMSLP and IMDBP so that pages on each other's sites can be easily referenced, e.g.
[[imslp:User talk:Pml]] or [[imdbp:Otello (Verdi, Giuseppe)]]
without having to specify the full path e.g. http://imdbp.org/wiki/Otello_%28Verdi%2C_Giuseppe%29
3a. You might as well enable interwiki links for Wikipedia and CPDL, also.
4. See below. The NavBox template over at Wikipedia uses a whole suite of multiply-related templates to do its work, so this might be a bit of work.
5. The Percussion wiki/unusual staging requirements info idea proposed elsewhere by David Brooks:
To be implemented in one of the (as-yet-unused) parts of the IMDBP templates, seeing as only the "basic information" one is being used currently.
6. Clarification on whether non-PD composers/works (no permission granted) can be registered over at IMDBP, i.e. "yes, we know this composer or work exists, but can't be hosted at IMSLP"
I'll be back later when I've got more time to add things in... :)