I can’t speak to exactly what the OP was thinking of, but it would make sense to me that if you have turned on the Watching First Post mode for a category, then the email could include a button/link to directly watch the topic. Underneath the Visit Topic button maybe there could be another saying “Watch this topic to be sent emails for all replies.”
Perhaps there could also be a little explanation sentence in the footer too: “You received this email because someone replied to you/you are watching the topic/you are watching the category/you have requested notifications for new posts in this category.”