I'm a very results oriented person. Doing shit for arbitrary numbers in class? Doesn't do anything for me, they're just numbers. However making something that I actually find useful will get me going. I used to take comp sci classes and know Java, even made a few shitty apps for class back in the day. However, I had a hard time sticking with it because we weren't really learning anything to make it immediately valuable. Could dick around in Php for weeks trying to build a website, but cannot focus for classes.
Anyways, my idea is to port PKHex, a popular open source Pokemon save editor. Someone made a port a few years ago, but it doesn't work with anything past Android 11, hasn't been updated in 2 years, and the dev expects you to build the app yourself. So making a new version for Android seems very interesting to me. Is it a good idea for a first 'big' project?
You can do both: develop from scratch and after that fix the old port so it works past Android 11 (and implement lessons learned from that into your original version).
What windows app?
PK Hex, it's a save editor for Pokemon. Lets you check EVs and such in older games and can act as a sort of Pokemon Home for people on emulators. I'm surprised there isn't someone else doing it already because of how big Pokemon emulation is on Android.