zcash
Modify ZIP 316 to allow inclusion of "use before block height" metadata in a unified address.
Key expiry is important. We should make it possible for the producer of a unified address to convey the information that they don't wish a particular address to be usable indefinitely, and that the address should be considered to be expired as of a particular block height. This could be readily implemented by defining a new range of metadata receiver types and specifying a receiver type within this range where the expected value is a block height above which the address should no longer be used.
@str4d notes that this could risk linkability of diversified addresses, so support of such a feature would need careful consideration. Perhaps we could take the same approach as we do with fees, and standardize on heights that wallets will prefer to use?
Also: a UA could usefully express a preference for V4 vs V5 transactions. Although it's likely that a wallet that produces UAs will support V5 transactions, in the future it's likely that we'll want wallets to communicate this information.