It might be nice to include a few negotiation utility functions either in this bip or at the same time in a separate bip. An example we might want to include is a "polite disconnect", whereby a node can register that you don't want to connect in the future due to incompatibility.

It also might be nice to standardize some naming convention or negotiation message type so that we don't end up with different negotiation systems. Then we can also limit the bip so that we're only defining negotiation message types as ignorable v.s. some other message type (which can also be ignored, but maybe we want to do something else in the future).

This also makes it easier for old (but newer than this bip) nodes to apply some generic rules around reporting/rejecting/responding to unknown feature negotiation v.s. an untagged message which might be a negotiation or something else.