I’m not sure if subscribing others is a good idea: just like how others can’t add pages to my watchlist, I might not want others to add topics to my subscription list either. If this gets implemented this way, at the very minimum users should have control over whether they can be subscribed by others (preferably in an opt-in manner).

The Teahouse at enwiki uses a bot to deliver messages. The messages are "signed" by an individual editor (selected at random, so it's not always the same person). It would be nice if the bot could subscribe those individuals to the messages it's delivering on their behalf.

I suspect that Growth's Newcomer work would also benefit from such a feature.