Although it is an exadgeration to say that app stores are dead it is true that web apps (applications that run in a web browser) and the mobile-device browser platforms have become powerful to the point that it is often not neccessary to build a native app.
There is a gap in the web app paradigm as users don't always realise they can install a web app they use by going to "Add to homescreen".
An interesting way to fill that gap would be to build an "app store" for web apps. That is, a native app that curates and carries out the "add to homescreen" process for the user for a wide variety of quality web applications. So basically like Firefox OS but without the OS - just the installer, and for the native browser of each platform.
Ideas are cheap and execution is everything - you don't need my permission to take this idea and run with it if you're convinced.