Let's go...
1. Trash left behind after an app uninstallation: files on sdcard, AppData and Obb folders, asec (encrypted) folders left behind, backup/lost apk files, libraries, dex/odex files. For this, the better will be a "resident" module that monitors uninstallations (oh! root will be needed for some of these operations...).
2. Duplicate files and (at premium?) ability to delete one of them.
3. Big files.
4. Must have: whitelist for cleaning. Right now, all files of RomToobox (for instance) are being detected as "trash", making the advanced cleaning impossible (and dangerous) for me.
5. Database alignment (and compression). Root needed?
6. Downloaded apk files.
7. Please, don't add the "memory" cleaning as Gb/Mb cleaned as a lot of silly apps... We're not cleaning anything but RAM, not files...
8. Privacy cleaning (history) and links for manual cleaning of SMSs, call logs, etc.
9. Useful task management: with whitelist, showing increased RAM and Temperature decreased, etc. (If possible, with root access).