> * Key-value map model or set model.
> * Ability for Combiners to verify two PSBT are for the same transaction
> * Optional signing
> * Derivation from xpub or fingerprint
> * Generic key offset derivation
> * Hex encoding?

I think all of Pieters points are valid and reasonable thought, though I’m unsure if it would be worth changing the existing-implementation-breaking things like the k/v set model.
AFAIK things like non-hex-encoding or generic key offset derivation are extensions and would not break existing implementations.

Further thoughts on BIP174 from my side.

Key derivation in multisig:
From my understanding, the signers and the creator must have agreed – in advance to the PSBT use case – on a key derivation scheme.
BIP32 derivation is assumed, but may not always be the case.
Sharing xpubs (the chaincode) may be a concern in non-trust-relationships between signer(s) and the creator (regarding Pieters xpub/fingerprint concerns).
Providing the type 0x03, the bip32 derivation path is one form of a support to faster (or computational possible) derivation of the required keys for signing a particular input.
From my point of view, it is a support of additional metadata shared between creator and signer and provided from the creator to the signer for faster (or computation possible) key deviation.

I think it could be more flexible (generic) in BIP174.
It could be just a single child key {32-bit int}, or just a keypath ({32-bit int}]{32-bit int}…) which is very likely sufficient for a HWW to derive the relevant key without the creation of a lookup-window or other „maps".
It could even be an enciphered payload which was shared during address/redeem-script generation and „loops“ back during a signing request.

Maybe I’m overcomplicating things, but for practical multisig with HWWs, a simple BIP32-child-key-index or BIP32-keypath derivation support field should be sufficient.
A generic „derivation support field“, provided from the signer to the creator during address-generation that just „loops“ back during the PSBT use-cases is probably a overkill.


Thanks
—
/jonas