Personally, I'm still waiting for the file browser app that supports folders. And in addition maybe tagging and commenting files and moving them to a folder.
I guess some Notepad/Todolist/Calendar functionality would also be appreciated by many. Or an RSS reader.
Regarding your idea...what emails are you talking about? For a full email client you could use existing POP3/IMAP clients and write a MUA frontend. I guess with KDK you would have to do all this in Java so you can't reuse fetchmail or something like that. Lots of work because of KDK limitation.
OTOH, if you're just talking about the eMails received from amazon to yourself, I don't really see the point. I just use the USB connection..
Another nice hack would be to use USBNetwork by default and add dhcp and file server to that, e.g. udhcpc and gatling. Then you can read docs while accessing the storage via ftp/http/smb.