OP_INTERNALKEY.

Would add a script instruction that fetches a Taproot coin's main key, saving space in some scripts.

Every Taproot coin has a main key, the 'internal key', alongside any backup scripts. Scripts that need that key today must repeat it in full. OP_INTERNALKEY would supply it directly, saving 8 vbytes (a measure of payment size) in some cases, and keep scripts in step if the key changes. It would be added by soft fork.