Devs make things they feel like making, or things they think the world needs.
What kind of an app (web, desktop, or mobile) would you want to see? What is something you wanted or needed for a long time that simply isn't there, or there but so bad with X that you can't use it?
Alternatively, what is a common problem that could be solved with software but hasn't been yet, or at least not effectively?
Leaving this vague on purpose, to let Tilderinos express themselves.