Here is my idea for Descendants 4. (A fan can always dream right?)
Mal, Evie, Jay and Carlos go to a summer vacation alongside their significant others. They have also invited Uma, Harry, Audrey, Tyler, and Chad with his girlfriend Aria who is Ariel and Eric's daughter.. When they are hiking at the woods, Mal and Uma feel a great source of dark magic, near them. They later realize that they are on bald Mountain and Chernabog's daughter Charlene, has awaken.