Hi! I might be able to answer your question. I work on the team that’s working on making it possible to develop applications for Endless OS.
The answer is that you can already do so by writing an application that works on the GNOME platform. These will run on Endless OS. For more information about developing for GNOME, check here: https://developer.gnome.org/
However, there is a problem: publishing your app. At the moment, even if you have written an app, there is no way to publish it in our app center. Your best bet would be to contact us and tell us why you think it would make a good addition to Endless OS, but I couldn’t make any guarantees. 
In the future, we would like to make our app center support installing Flatpak apps and Flatpak runtimes from other sources, so that publishing becomes a lot easier.