OP_TXHASH.

Would add a script instruction that lets coins check details of the payment spending them, enabling new kinds of spending rules.

Today a Bitcoin script can't easily look at the payment trying to spend it. OP_TXHASH would let it pick out chosen parts of that payment, such as its outputs, and fingerprint them. Paired with a signature-checking instruction, this could support today's signing options and many new ones. It would be added to Taproot scripts by soft fork.