I thought of a great idea. When you go to the end your first time, you don't go directly to the end dragon. You go to a nearby an end city or in one of the random areas you must then find 9 "Guardians castles" and fight the "Dragon Guardian" they drop their crowns if you defeat one. Once you get all 9 of their crowns, you head to the "Dragon's chambers" where you have to put all 9 crowns on pedestals to open a real portal to the end island. And you can't reach the end island from the outer end islands because there would be a barrier around the whole main middle island until you unlock the main portal and go through it.