Can someone give me good ideas of useful apps ? Preferably ones that i can use free api(s) ( i want to make it open source) . And please don't say todo app or weather app.

not my Idea, but I like it:

For the cold month:

A handwarmer (Web) App !

User opens app / site with smartphone, sets how warm he wants it (number of threads / CPU utilization) and gets a warming phone to heat his hands. Maybe some points also...

You make bling bling by mining crypto on users phone....

Where did I park my car?
App in wich you can mark where you parked your car. Gives you the option to learn how to work with Maps and gps.

Trivia app
An app in wich you answer questions for points. You might add multiplayer with friends. Newseum.org has an api for this.

Kitten app
There is a kitten api out there. Make an app that can track your mood with kittens.

My ideas are not getting any better. You could also browse https://stilist.fun to see if an api might give you and idea.

In line with @MisterArie

An app that helps you study, so for a topic it will pop up a notification question for you to answer and a notification concept for you to learn. Frequency specified by user like 1 question per day or something like that.

And you can upload all your material, so you can study your own topic.

I see my link was incorrect xD

Https://apilist.fun