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?
A sort of escrow could be 2-of-2 multisig where seller puts one price and buyer puts two prices. But for it to be efficient both funding transactions should be included in the same block. Meaning that besides the usual transaction hash its counter-party transaction hash also should appear in the Merkle tree.