I don’t think this makes sense for the use case being addressed by this snap. A strict snap would not be able to prove domain ownership and configure HTTPS automatically for an HTTP daemon running outside itself. Creating an interface for such a thing would involve embedding or re-implementing certbot inside snapd’s implementation of the interface, which doesn’t seem practical.
It would make more sense for there to be a certbot part that can be added to confined snaps that provide HTTP daemons. I don’t see any reason why such a part couldn’t be maintained alongside the certbot classic snap (which would continue to exist for the “HTTP daemon running on traditional host system” use case), but I don’t have any plans to work on that side right now.
It’s been mentioned that the nextcloud snap already does something like this.
See Certbot: request for classic snap approval for some previous discussion on this.