One thing on my wish list - for this BIP, BIP 88 (Hierarchical Deterministic Path Templates) or yet another one - is to include a birth date (minimum block height). E.g. tr([m/86'/0'/0']xpub.../{0-1}/*)>709631
And then of course there's the gap limit. Perhaps we just need a "metadata" format to go along with descriptors to track the birth data, gap limit and anything else you need (nonce collection for musig2 setup?). E.g. a simple dictionary: tr([m/86'/0'/0']xpub.../{0-1}/*){dob:709631,gap:1000}
The UR standards we use in Airgap Wallet Community for interoperability (currently used by a number of recent wallets for airgap PSBT via animated QR) leverages CBOR's tagging capability, and thus explicitly supports metadata. In particular the spec reference code support optional seed birthdate as some wallet vendors really wanted that metadata.
It would be trivial to support it for hd-keys, and if gap is important, we could also easily add this to the hd-keys spec as well. That is part of the reason why use CBOR for the underlying binary encoding is tagging lets us add important metadata. The UR standards themselves adds to binary CBOR encoding a very efficient transport via QRs or URLs that leverage native QR compression.
See:
* Articles: [URs: An Overview](Docs/ur-1-overview.md)
* [A Guide to Using URs for Key Material](Docs/ur-2-keys.md)
*[A Guide to Using URs for SSKRs](Docs/ur-3-sskrs.md)**
* [A Guide to Using UR Request & Response](Docs/ur-99-request-response.md)
* Specs:
-- Christopher Allen