I'd really like to see dynamic spawning implemented in the End. Many players including myself like to go there to skills and vanilla XP grind, but it doesn't work very well if anyone else is in the world. This creates a problem because they either spawn near you, or the other person almost exclusively and continually. People build Enderman grinders, but again someone will be getting the short end of the stick, with Endermen spawning exclusively at one place or the other. If it were dynamic to the players then many people could be in the End, either on the main land or a grinder doing their own thing without being slowed down by other people being in the End.
The other suggestion, borders. Make it so that they can only fight Endermen on the main land, where everyone will have to share, then there would be no arguments or complaints about Endermen not spawning.