I cam across this neat plugin. It's a follower system, sort of like Twitter or Facebook, but for MyBB boards. I thought this would be unique as I have never seen this on any other forum.
Unless the settings can be extremely tweaked (e.g. Only receive a PM when {user} makes a post in "X" section", only receive a PM when {user} posts a thread instead of a regular post), I really wouldn't want to see this be added. It'd get annoying to receive constant PMs like that.
If this were to be added, I wouldn't be using it. .-. That's just me though.