Thank you for taking the time to share what comes to mind, @Sdkb. A couple of things you said prompted new thoughts and questions. I'm going to quote and reply to each below.
Before that, I wanted to clarify: as we are currently designing it, the new feature we are working on will *not* affect the way the Watchlist functions. Instead, it will introduce a new event for which people can receive notifications about via Echo. If this information brings new thoughts to mind, I'd be keen to hear them. In the meantime, I'm going to respond to the points you raised...
Something that's always good to have in mind is how a technical change might affect editorial culture....not getting notified about other discussions on a page I'm monitoring for my own thread might mean slightly less branching out, and fewer replies to threads in some areas.
Accounting for the unintended consequences increased specificity could have is a great point. I've made a note of this on Phabricator here so we can think through how we might monitor/evaluate the extent to which what you are describing above is happening: T274215#6845061.
One loose idea/metric we could use to track the move: "Of the pages on which people are subscribed to at least one section, how many of those pages do people also have on their watchlists?" <-- if other ideas strike you, please comment them.
It could also introduce new avenues for abuse, such as someone starting a new section on the same topic because they suspect an opponent is only monitoring the original section and don't want them to notice and reply.
Another a good thought. I've also documented this on Phabricator here: T274215#6845061.
Talk page sections can be fluid, where sometimes they'll be created in a discussion that's already well under way, or sometimes threads on the same topic will be merged.
Can you describe this scenario in a bit more detail? Or better yet, link to a page/diff where you've seen what you're describing happen?
This might be a little overly fancy, but sometimes, there are pages where I might want to pay attention only to future sections that meet or exclude particular criteria.
This idea of enabling editors to be able to write custom rules for the kinds of activity they are and are not notified about strikes me as something that could be, as you said, quite "powerful." While it is outside of the scope for what we are able to take on as part of this project, I've created this ticket in Phabricator so we don't lose sight of it: T275260.