#2 Nakamura is not ready
Shinsuke Nakamura will be the main event star and he and AJ Styles will tear the house down when given the chance, that chance though should not be now.
Due to underwhelming feuds with Corbin, Mahal and ironically even Ziggler, Nakamura really has not hit the ground running like most would expect. The fact is he has not come across as a main event player on SmackDown and has no real buzz around his name at the moment.
One solid year of booking can change that as he has charisma by the bucket load. That year will probably begin after Wrestlemania and will be an amazing match at Summerslam or perhaps at the next Wrestlemania.
Lastly, Vince loves Heel vs Face on the biggest stage and a face vs face encounter is very rare in a title match. At the moment it would make no sense for either man to turn and apart from Owens and Zayn, Ziggler is the only real heel that could put on a 5-star match with Styles.