Tail Call Execution Semantics.

Would let a script reveal and run only the spending rule actually used, out of a large set fixed in advance.

Earlier upgrades (BIP 16 and segwit) still require all of a coin's spending rules to sit in one script, used or not. Together with BIP 116, this would let coins commit to a practically unlimited number of separate spending paths and reveal only the path taken, which saves space and keeps the others private.