Nice to have non yet existing applications on the blockchain?

[tyz]

A friend of mine is a good programmer and he told me he is busy to learn the nature of blockchain. He wants to realize a small use case on the blockchain and he ask me to help him. The use case itself is nothing for mainstream use, just a prototype which probably will never be used by anyone But it is good to learn how to make applications on the blockchain.

That's why I've been thinking about what (useful) cases and applications have not been implemented on the blockchain yet. What do you think?