I may suggest some sort of simplish video editor, that for example replaces the audio of a video track with a mp3 on your device, that can cut/attach different video tracks with maybe a couple effect, extract a frame from the video, add written things on the video and so on, some basic video things, so that you can edit a very little before posting on youtube or somewhere else. It's not such a simple application probably, but well, I tell it, and you do what you want with it.

A file searcher and application launcher would be nice too (à la Google Desktop/Spotlight) as allready stated.

Is batch renaming & resizing & converting of images allready there? otherwise that too.

Mms support (rarely used, but well, we strive for perfection ^^).

An application à la PhotoBooth that makes use of the front camera or back camera (or both for some effects?). (You would be surprised on how much people loves that stuff.)

An application that checks my email box via POP3/IMAP and collects only the headers of eventual unread emails, and removes them once read. A sort of very light email reader that doesn't require to have data stored on the device. (or improve the existing mail application to support such a modality)

Just brainstorming, if more ideas come, I'll post them.