Here's a good idea about joining or not joining a dual or some players call it a fight. How about if Wizard 101 made a spell that looks like a gold rope that wraps around the fighting area. When the spell is cast players can not join the fight. I see players all the time getting mad at others because they join the fight just when the fight was almost over. Just telling someone please don't join does not always work so I think this gold rope spell would be a good idea. Let me know what you think.
I think I have to disagree with this. The image I got in my head after reading your post was a whole bunch of locked out duels on the streets leaving basically no room for anything else, or new monsters. Especially on busy servers.
I think the way this whole encounter system is set up is probably the best that you can get until the human species learn how to grow and evolve.