Which of WWE's current roster can be considered a legitimate main event attraction?
The old-timers don't count. As if it wasn't self-evident enough, the fact that Goldberg et al. are booked far more favourably than their younger, full-time counterparts drives the point home. John Cena, obviously, is a headliner. Randy Orton has spent more time in the midcard in recent years, but he is a proper star. As this year's WrestleMania attests, he's never far away from the spotlight. Roman Reigns might not be widely accepted as a headliner - but he is a headliner, nonetheless.
It gets trickier further down the totem pole. AJ Styles has headlined more than one B-level pay-per-view, but there was never any notion of his going on last in Orlando. Being the recognised best wrestler on the planet doesn't mean anything, sadly. Bray Wyatt is the current WWE Champion - but WWE is notoriously capricious. There's every chance he'll enter a supporting series with Luke Harper when he relinquishes the gold to Orton. Chris Jericho is a chameleon. Seth Rollins continues to struggle for acceptance in the headline role. Finn Bálor is unproven.
The role of headliner is amorphous, subjective, but using the last match at WrestleMania as the barometer - and why not? - there are very few men WWE would trust in that position.