OK I get it.
Perhaps I’m coming from the technology angle on this.
The reality is it though, it would be foolish to write this without making them some form of User (technically at least), because so much existing logic could be used if you did that.
e.g. the “Watching” logic that kicks off when an update happens on a Topic is going to check for all Users that are watching that topic. You’d be mad not to leverage that, not least because anything you write will have to be supported going forward and if you use the core logic, it’s maintained for you!
Perhaps this IS a presentational thing where you modify the front end (and back end where necessary) to take an email and register it as if they hadn’t been made an official user of the site.
In reality, and technically, in the back-end this is registered as a new User, but you use some way of setting them apart - I wonder if not activating them might do it? There are various changes you’d need to make to make that work but it might be doable.
If you are really interested in building this and have budget, I’d post in #marketplace.
It’s potentially quite a complex plugin, but an interesting one! We’d definitely consider it