i personally am very much in favour of making a gradual move towards this by suggesting that relays and clients with some nip-11 flag understand events that are in canonical format, and then you can add a futher tag with a list of supported encoders and the in-event signature for other formats, which are optional, only the json form is mandatory, can have signatures for your faster verification form the binary form. done.
Discussion
also just need to point out - the signer of an event could make an after-the-fact amendment to add the tag with another tag that refers to the replaced version that gives you your new (multiple) signature tags as i described, much like the parameterised replaceable protocol already existing