Currently, bitcoin.Transaction may contain per-input witness data, and that witness data is an array of Buffer objects.

Similarly, bitcoin.Psbt can have a finalScriptWitness field, but that one is not "destructured" (for lack of a better term), but rather lives in a semi-serialized state with the encoded witness stack size concatenated with essentially a bitcoin.script.compile() of the actual witness elements.

I think it might be incredibly convenient to have the object represent the destructured witness elements. This would have the added benefit of allowing the construction of partially finalized PSBTs where some of the inputs are non-standard scripts.