This app is a tricky idea to work around. Having a place where you store all of your passwords on your phone, something you could lose or it could be hacked into. So creating an interface that requires a thumbprint or retinal scan to unlock it. A backup system would also need to be employed to save passwords in case the owner lost or broke their phone. As far as themes, a vault / safe design could be cool be probably has been done before.
Halo 5 App
The combination of video games and apps has been a popular trend recently and i personally feel this would help create a more complete experience and give another avenue of potential income. Halo 5 is one of the flagship games tied to the Xbox franchise and has other games with app tie-ins. 343 Industries (developers of Halo after Bungie left) have included a requisition system that gives players cards associated with boosts, weapons, and vehicles in the game. The options for an app like this are endless, with the the opportunity to integrate customization of the users gaming profile, forum boards, and the new Spartan Companies launched with the game as well. This would allow the user to message the members of company and manage it from their phone.
eSports News App
eSports is one of the fastest growing sports and has a huge following across the world. I have witnessed this personally using the popular sports app Team Stream with my news feeding being invaded by teams and people i have never even heard of before. This could easily be translated into a more genre focused app providing highlights and stream features to watch live events on the app itself. Features could include message boards, following favorites on your news feed, customization themes for the app mimic popular games like LoL, Counter Strike and Dota.