Alby the browser extension. We need better wallets and better ways for people to use their coins. Alby is following in the steps of Metamask which AFAIK is the most successful crypto wallet out there, and we desperately need something like this in bitcoin.

LNURL and all the tinkering that is going on in that community

Taro - It's not well understood by everyone yet, and still in conceptual phases. But if it works as I imagine it doing it could disrupt massive parts of the crypto industry and solve many of the problems that we currently have.

Chaumian vaults sounds really interesting too as a model that offers interesting tradeoffs between usability and decentralisation.