I was thinking that maybe the server could add in a shackles plugin or something that would make a player be able to take another person prisoner. I thought this could work as someone right-clicking another person with, for example. iron bars, which then would mean that the player loses all control over where they can walk or do. The person holding the prisoner could then tell them where to go. A possible way to do this would be by using a fishing rod or something and then having them be towed around or placed somewhere. For example, use a tripwire hook to keep them stationary. Plus it would make jails a lot more useful.
It would also make it a lot more realistic in wars. To release them a player could either give them an item or right-click with, for example, shears.