Zack felt bad about making Susie feel so embarrassed. He decided to make a wish that she wouldn't feel embarrassed anymore when he realized: if it were normal to act like she did, she would have no reason to feel embarrassed!
The way Zack saw it, he could make the wish two ways:
One way, he could make this normal behavior for teenage girls. That would mean that girls would be really far behind guys developmentally. That would definitely have huge ripple effects in society. But then again, Zack also started to get excited at the thought of all girls around his age walking around in diapers with thumbs and pacifiers in their mouths.
On the other hand, he could make all teenagers be big babies like Susie. School and such would just start later for everyone, giving him and everyone else plenty of time to enjoy diapers, baby food, and naps before having to grow up. However, seeing as his sister was as mentally regressed as she was at age 18, how regressed would 16-year-old Zack be?