What "killer feature" do current study apps miss?

Hi everyone, I am the developer of Brainy Learn, and even though the app has many great features (Google the app if you want to learn more), I've been looking for features that many apps are missing today. Something like killer features, the app is still in the early stage, but production stage, and still has the potential to incorporate some killer features.

Which feature would you like to see in a free and open source app?

The one I've been looking at is incremental reading; no new good software has that, at least no one built it right, it seems like.