Okay, assuming that Dwayne's schedule is clear and he can get through whatever insurance/legal hoops are in the way, the obvious main-event for “WM39” is Roman defending the unified titles against Rock. Two-and-a-half year champion, biggest celebrity on Earth, they're related, it's in LA, etc. It just makes too much sense.
But is it actually the right way to go? At this point in Roman's (heh) reign, the only time that seems appropriate for him to drop the titles is in a WM main-event. Maaaaybe “SummerSlam” if the challenger was on fire and booked extremely well, but Roman shouldn't lose them at some C-level event. So then, what, Roman beats Rock and then holds the title(s) for ANOTHER year? And on a part-time schedule, at that?
Shouldn't WM39 be the point where Roman makes or cements a new main-eventer? McIntyre, Lashley, Riddle, Cody, Wyatt, Strowman… hell, Jey, Zayn, Butch, Logan, McAfee, Breakker, Bate, take your pick.
And just friggin' shoot me if Rock actually wins. Dude was running on empty and held together by duct-tape a decade ago (not to mention that all of his promos had basically declined to “you're the gayest gay that ever gayed”), we don't need an even worse version of that.
So who do you think should headline WM39 against Roman? Rock, because it makes sense and will draw big in the short-term, or someone else in an effort to make/cement that person so that they can draw even more in the long-term? Roman/Rock absolutely should happen at some point, and should be done on a big stage for big money, but I think it could easily be done at SummerSlam or one of the $audi shows, thus clearing them up to help other wrestlers at WM.
But you don't really NEED a big headlining match when the show is already close to selling out and PPV numbers are meaningless anyway. Hence the idea that Cody Rhodes should be the guy. Or it would be kind of funny if Bray Wyatt got heated up to the point where he could challenge and then regained the title 2 years after being the guy to lose it to Roman in the first place. Either way, I just don't see the need for putting the Rock in that spot.