What should I learn next after mastering a Flask to-do app?

You've got a solid base with Flask! A good next step could be to create a URL shortener. It's a fun project that will help you work on important concepts like routing and database management in a real-world context.

If you're looking for something more advanced, consider building a Rust WebAssembly frontend for your Flask app. It will give you a taste of how to integrate new technologies and improve performance.

You could also think about adding real-time features to your to-do app, like a small chat or real-time syncing using websockets. It's a fun and engaging way to push your skills further!

Or just take on a wild challenge! Someone wanted to hire me to clone Amazon, and while that's probably too ambitious, starting with a smaller project could be a blast!