The usual formula is that Raw's world title match is the main event. That, of course, means the Universal title match between Brock Lesnar and Roman Reigns. The AJ Styles vs. Shinsuke Nakamura match is supposedly the dream match for all the indy/Japan marks.
Lesnar and Reigns have unfinished business from WM31. So this match is three years in the making. But, if the rumors are true, Lesnar is going to do the "time-honored tradition", lose his belt to Reigns, and head off to UFC again.
For that reason (plus the Royal Rumble angle), the WWE world title match should be the last one. But, with WWE and their investment on the two big men, who knows?