There is definitely one smart contract I'd like for Bitcoin: escrow.
OP_2 OP_3 OP_CHECKMULTISIGVERIFY
This "smart contract" has existed in bitcoin for a very long time! you can also add additional conditions to it such as time with a locktime (OP_CHECKLOCKTIMEVERIFY)
is there anything specific you have in mind?
Or for inheritance (if I don't sign my wallet for 7 years, it releases to an heir, for example).
OP_DUP OP_HASH160 OP_EQUAL
OP_IF
OP_CheckSig
OP_ELSE
OP_CheckLocktimeVerify OP_DROP OP_DUP OP_HASH160
OP_EQUALVERIFY OP_CheckSig
OP_ENDIF
if "you" wanted to spend it, you simply provide a using your own private key. if your heir wanted to claim this they have to first wait for the locktime to be reached then provide their own .