I know it's a bit pointless making suggestions here, because really if one want something in Rockbox one needs to be technically competent enough, and prepared to put in the work, to do it oneself. Keep wishing I could magically acquire that degree of required competence because there are quite a few things I'd like to add if only I were smart enough (the big problem, I think, is being able to understand the existing code-base).
But just in case someone capable of implementing it happens to also like the idea, I'd quite like to see a plugin that, perhaps re-using the same database that the existing pictureflow plug-in builds and uses, allowed you to browse albums via a kind of grid-view of thumbnails of the cover art. Much as I think, if I remember rightly, various Sony players allow in their original firmware.