At my work, we need this feature. Basically, UI where a user could check/uncheck categories, and automatically receive emails for each new post in the selected categories. Similar to the emails you get once you start watching a thread, but starting with the first post.
I’m willing to build it and submit a PR (if the core devs think it would be useful), but I thought I’d check in first to make sure (1) it doesn’t already exist somewhere yet, and (2) perhaps be pointed in the right direction on how best to implement it.