I suggest an improved media library, because this is a feature of WP which has been strongly neglected in the past. Whenever someone came up with a new proposal, most of it went under, unheard or was post-poned till somewhere in the far-off future.
At least having some option to display files and directories in a classic tree view would be great. Oh, and a slightly more welcoming Media API.
Both features would be very welcome. The former one should sooth newcomers, who are used to having a "regular" overview of their media, while the latter should rush forward plugin development.