I am not going to look a gift horse in the mouth because I love this series and am excited to get more. But I was very very happy with America and Maxon's ending. They're perfect together. I am very worried that this book will include their relationship changing and I'd rather it end at 3 books then have their relationship change or go haywire. I hope this is about their kid or like another person posted, about some Princess Diaries type hijinks. Considering the title is "The Heir," I'm assuming it will do with the child. Maybe the child is a teen now and going through their own struggles with being the heir and whatnot. As long as there isn't like a kidnapping involved. Don't do that to America and Maxon. Make their kid happy and safe. Don't make this some Twilight crap where someone wants to get the kid to destroy the happy couple. 
 Update 10/25/14 
 Holy cannoli. Read Cass' blog post on The Heir. I am so excited. America and Maxon had twins! And their daughter is going to have a selection. So exciting! I'm really looking forward to this. I was worried about what would happen after the happy ending of the third book, but I'm really confident that Cass is going to make it work out really well.