@LPardue just raised an interesting point on the mailing list, which is currently not covered in the documents:

In draft-deconinck-quic-multipath-03 [1] it states that because there are new path-related IDs, some frames need to be modified and then lists them: NEW_CONNECTION_ID, RETIRE_CONNECTION_ID and ACK. IIUC this would keep the type value but modify the frame contents. My model of thinking has been to assume that frame definitions are immutable, and that extensions that require tweaks to frames would define new ones.

[...]

So my observation is not about the proposals themselves but about the composability of extensions. Say for example someone wanted to combine mpquic with 1wd, would they modify the ACK frame or define a new one?

I find the composability argument convincing, and was wondering if it makes sense to add something along those lines to the transport document:

Extensions MUST NOT modify existing frame types.