You forgot Turkey as well. Yes, the EU is open in general for specific treaties. If possible they want to do broad treaties (see the EEA). But Switzerland is almost as big a partner as the UK. If it is big enough, they'll do custom deal.

There is not much trade with Canada at the moment but with some special treaties this might change.

Then there's the political edge to this "associate member" status and if it can work as a blueprint for more countries, they'll certainly will be open to do it.