Something that may require some thought if you want to do this is how you handle import/export of documents with embedded scripts.
You'll need to be very careful not to fall to the "applications that download additional code" rule there. This happened to Codea (http://twolivesleft.com/Codea/Talk/discussion/comment/14504#...), and afaik they still haven't managed to get their project sharing feature re-approved.
Speaking of it: Support for WebDAV, SFTP, box.net, SkyDrive, SugarSync, AFP and SMB would be great too! (In this order.)
Otherwise a proper scripting language in a spreadsheet app on my iPad seems very promising. It would be also great if one could search public scripts of other users.
