I assume that Generation Z will be mostly Democratic because this group is more ethnically diverse than the last one.
I could also see this group being moderately more Republican than Millennials (but still overwhelmingly Democratic overall) because these people were too young to remember Bush, so they'll hate GOPers significantly less.
What do you define as "overwhelmingly Democratic"? In 2012, millenials (age 18-29) voted 60-37. I don't consider anything less than a 20-point margin to truly be overwhelming. Do you think Gen Z will be 55-45, or 58-42, etc.?