If we're gonna talk about an 'epic finish' character, there are only a few I think would fit:
Master Chief would be the most obvious in my mind. Every thrid party so far has had a lot of Nintendo history, or been one of several PlayStation 'mascot-esque' characters. All Eastern. So far, the only Western characters we got are Nintendo first-party characters made by a second-party studio in a Western-focused-and-therefore-kinda-forgotten franchise, or the third party character who used to be from one of the aforementioned second party studios before being bought out. Speculation has really focused on Japan. Thus, a Microsoft exclusive character, exclusive to a console (!) that isn't really selling to any major extent in Japan (!!) from a genre that Japan tends not to favor (!!!), yet still being something incredibly recognizable, is a feat in itself. I really think that the Chief would bring the biggest 'WOW' factor to Smash since Cloud. He's really the only big one I see: Scorpion or Doomguy hit many of the same points, but I'm docking a few points from them since they don't have the 'exclusive to a direct competitor' status. Also since both have gotten recent games on Nintendo as well, so they're a little more in the realm of possibility.
The only first-party I'd consider in this place is Waluigi, and that's just for being the perpetual loser in Smash. It takes talent to have mainstream news articles written about getting snubbed again. It'd be less impressive to see him make the jump if Spring Man makes it first, but let's face it - you'd be hard pressed to find someone who doesn't know who Waluigi is.
Finally - and I know people will hate me for even thinking it - if Sakurai wants to blow people away with who he's adding, he could go back on his words again and add the one non-videogame character who's memetically synonymous with troll Smash picks: Goku. Seriously. I'd hate it as much as most of us here, but I was talking to one of my coworkers way back when the DLC first got announced, and he was even saying how mindblowing that would be. This is a guy who didn't recognize 4 out of the 5 DLC characters we got, but even he knows the memes. If they announced that, these board would implode.
But let's face it: this is Smash, and Smash will have its blowout characters early. I'm expecting the biggest characters to by probably packs 7 or 8, maybe 10 if they want to have a blowout for next year's E3, but you know that character 11 will be a total whimper. I wouldn't put it past them to drop a trailer for Fire Emblem game #17 around December, release it in July, and have it be just timed out so that blue-haired swordsman can get in as FP11 in December 2021. /s