Here's an idea...
Don't reinvent the interface wheel (like all the other apps out there). Build on the Apple interface and create a app that works and looks like Apples "photos" or "Ibooks" but combines the two apps and adds functionality. Keep the Apple "feel".
Must keep the ability to view thumbnails like the Apple programs (nothing I've found does that)
Add file management functions..delete, duplicate, move, etc
Add ability to "open in" interchange features.
Add FTP, evernote, and web storage support.
Add some minor PDF editing function, maybe delete pages, or combine PDF, but leave the markup business to other existing programs.
Add some minor photo editing - crop, rotate, adjust brightness, etc
At the first phase I wouldn't add too much creative control. Mostly make it a viewing program.
Strangely, many of the apps I find seem to try and take the Apple "feel" and push it aside to implement the MS-DOS and Windows XP feel for file management.
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Here's other one for you...
How about a program launcher?
Since Apple won't let me see more than 20 programs per page, I'm constantly flipping back and forth. If I could control the icon spacing I could easily double that number.
How about an app that lets me launch other apps. Create and app that let's me control the icon size, position, sort order etc.