There is definitely one smart contract I'd like for Bitcoin: escrow. We're talking about Bitcoin as a payment tool and (trustless) escrow has got to be one of the things left to develop properly for Bitcoin. I remember first reading about Rootstock (mentioned by Pooya) at some point even in 2016/17 said that they were developing it for that precise usage. Think even Counterparty was supposed to have something like this.

A simple Ethereum script for it has existed for some time. But not for Bitcoin (to my knowledge). I can see that smart contract being put to use daily on this forum, for example. Or for inheritance (if I don't sign my wallet for 7 years, it releases to an heir, for example).

Or do these already exist?