This idea is part of a group of ideas related to weddings I had back when I got married and the following years. Not sure if they still relevant, but I hope it can help you find new ones.
Let your wedding guest share their pictures and build their own wedding book.
When we got married we noticed that other than the wedding photographer, others brought their expensive camera and took pictures of the ceremony and party. When we met with our photographer we were not happy with the wedding book he made for us and ended up building our own with his picture instead. We wanted to include some pictures from the guest and it was very hard to get pictures from everybody.
This gave me this idea, why not make it easier for the guest to share their pictures. Maybe they would want the wedding book we built with them too or even make their own like we did.
Why not give the guest a disposable camera to take impromptu pictures of their experience. Then, they give back when they leave and the couple can use the picture for their book and/or site. Or apps that let them do this from their phone (less expensive than the camera) and let the app automatically share the picture or let them choose the picture to share.
I know there are already apps like this. A few years after our wedding a couple of friends got married and they sent us a link to an app for their wedding. It was kind of doing that and added a part to notify user of the wedding before the wedding. However, it didn’t have the part where you make your own wedding book. They went bankrupt, so I’m not sure it’s easy to find a way to make money out of it.
How could it make money ?
Why this app was more a convenience that I saw from firsthand experience here how it could make money. The couple and the guest could pay for the souvenir for the wedding book created after, they could buy prints of the picture. Perhaps couple the app with a gift directory where the guest can send gifts to the couple.