I realize the tagging system needs a bit of work, I think especially a follow functionality for tags would be nice. So for example you could follow "antimatter" and have the new papers tagged with "antimatter" automatically come to your activity inbox. It currently works like that for users, papers and comments. So if you follow a user, if that user posts a new paper or comment, you will get an update in the activity inbox in your profile.
Is that similar to what you had in mind?