A cryptocurrency wallet, because then you get to work with the ledger itself.

Go to meetups related. Talk to people. You can get many ideas on what is interesting and what problems there are. Then choose one and build a fast mvp. Then go back to meetups and show them the mvp.

Start with a problem you’re having in the space. What about cryptos annoys you or takes time that shouldn’t. Then solve that problem. If you haven’t found a problem, you aren’t deep enough in the crypto world yet.