This is a BIP to allow the spender to choose one of multiple standard scripts to use for spending the output.

https://github.com/TierNolan/bips/blob/bip4x/bip-0045.mediawiki

This is required as part of the atomic cross chain transfer protocol. It is required so that outputs can be retrieved, if the process ends before being committed.

https://bitcointalk.org/index.php?topic=193281.msg2224949#msg2224949

The script allows multiple standard scripts to be included in the scriptPubKey.

When redeeming the script the spender indicates which of the standard scripts to use.

Only one standard script is actually executed, so the only cost is the extra storage required.

A more ambitious change would be a soft fork like P2SH, except the spender is allowed to select from multiple hashes. Effectively, it would be "Multi-P2SH".

This gets much of the benefits of MAST, but it requires a formal soft fork to implement.

If there is agreement, I can code up the reference implementation as a PR. The multi-P2SH might actually be easier.