Since I’m not a blockchain, Cardano or security developer myself, I’m not sure if it would be a suitable project to get started, but I’d like to bring your attention to the post I made a few days ago: Integration into AirGap Wallet/Vault
I think the project is a really nice alternative to hardware wallets, especially for people with limited funds. Of course, it would be very interesting to hear your opinion as a security researcher about it.
When I read your post I thought that integrating Cardano into a wallet could be a good way to get started. But maybe I just vastly underestimate the work and experience that is required for such a task. 