I have also had a simple app rejected, then approved once some complexity was added. It was a totally silly app that was pretty much an exercise in what exactly they would accept as the simplest app. (https://itunes.apple.com/us/app/iuplift/id469338399)

Initially, the app was just showing canned upifting phrases. This was rejected. Second submission allowed the user to add their own phrases, using some native controls to show/edit this list of phrases. This was accepted.

What I would suggest is perhaps adding something like a way to manage different collect links in the app. Keep track of which photos you have sent to a collect. They seem to also love you to do things that are not easy to do in a web app. Maybe if you added camera access from within the app (useless I know, but still), maybe image cropping etc before upload? Is there metadata attached to their submissions to the collect link? Perhaps allowing them to edit this metadata in a table view.

I think you need to come up with reasons why you can't just make it a web app and want it to be an app (why do you want that anyway?) and be sure to implement those things.

My reasons for wanting my apps to be apps and not web sites is because phone apps are cool and people want an app not a website - it seems weird to me that apple do not think the same way. I understand they don't want any more fart apps, but most phone apps could be reasonably done as web apps now anyway.