-A detachable wired button pad for turning pages (at least).
-Highly user-configurable (fonts, margins, button actions, zoom levels). Providing some sort of .ini file or similar for interested users would be great (no need for graphical interface for every possible setting!)
-Customizable applications too. Users could add readers for other formats, different image viewers, calendars, games (sudoku, cross-words... no shoot-'em-up)