I just had an Idea for a mod where you can restrict players from doing certain things until they have met a few requirements such as killing a boss or getting gear. maybe even a custom leveling system. It would be great for mod packs but I'm not sure how hard it would be to make.
It could be used to stop you from going through portals, wearing certain armors, and right/left click with certain items. Maybe even a few other things but you will have to suggest.