Well, I suppose the first question to ask is what qualifies as a "headliner"? Because there are guys who've main-evented shows and PPVs that have never even sniffed the world title, and there are others like Roddy Piper who technically headlined WrestleMania but in my mind, is on a similar pedestal to guys such as Jake Roberts, Ted Dibiase, Rick Rude, Perfect, Davey Boy, etc. who are all well known, respected figures, with lengthy runs, but not necessarily what I would classify as a headliner, and that perception largely comes as a result of having never carried the title.
Then again, I ask what makes a headliner? Is it size, is it charisma, a combination of the two? And what about the era you wrestled in? In the '80s a guy like Rey Mysterio wouldn't have come within a mile of winning the title. So is he a product of his era? His move set and ring gear are flashy, but does he really have charisma? He's not much of a talker, and he certainly doesn't have the size. Yet, for a time I would've considered him a headliner. Which I guess would mean you don't really need to be a total package, you just need to be a product of right place/right time, and good booking. Size and charisma could help get you there faster, but depending on extenuating circumstances like roster depth, you may not even need those.
With all that in mind:
Charles Wright aka: Papa Shango/Kama/Godfather. With the right booking, I could see it, in the cartoony early '90s Shango could've beaten Hogan or Warrior. As Kama he certainly had an imposing look. Godfather not so much, but the attitude era had crazy booking, so I could've just easily seen it there too, he was very over.
Ken Shamrock: Like so many guys in his time, he was really a victim of circumstance. Without the explosive careers of Rock, Austin, HHH, and Foley, perhaps there would've been more room for him at the top. He certainly had the look, and in-ring ability.
Hardcore Holly: With a nearly two decade run in the company, and respect in the back, clearly there was something there. He had the size, probably could've tweaked the look a bit, but with the right booking around 2002-2005, while the current crop of stars were just beginning to get their footing, and the attitude era stars all on their way out, I easily could've seen the stars aligning to give Holly a brief run at the top.