VF 1-Lau
VF 2-Akira
VF 3-Kage
VF 4-Kage
Something interesting I find is that Lau and Akira have little to do with the Syndicate storyline, whereas Kage was fighting to rescue his mother. But, they all share one thing in common, being members of the original 9 character roster! Therefore, it can be deduced that the choice of a winner is biased towards characters from VF1, and in half of the cases, also relevant to the plot. I feel that's VERY useful for figuring out who'd win since it eliminates (or at least greatly lowers the chances for) about 75% of the cast.
But, ultimately IMHO It depends on if VF6 would expand on the oft-ignored story or not. At this point in the saga, where there have been 5 tournaments and with the Syndicate having pissed off so much of the cast (Kage, Jeffry, Wolf, the Bryants, Vanessa, plus Goh being targeted for assassination if he fails again, Lion's mistrust of his father, and Shun Di's search for the missing student leading the last few to naturally distrust them, and Akira probably suspects something is off since he canonically fought Dural), it would only be logical for this to be when the story starts having status quo changes. Maybe the heroes will go after the J6 head-on instead of just waiting for the next Tournament again.
(IDK how canon it is, but there was an, shit you not, pachinko machine that had actual cutscenes and story! It depicted Pai as being targeted by the J6 for...some reason. So add her to the list, maybe.)
In that case, I'd peg either Sarah or Jacky, since they fit the categories of being tied to the plot and from VF1. Plus I recall hearing somewhere, that Jacky didn't actually lose the second tournament, he just completely dropped the thing when he saved Sarah, didn't even face Kage. I'd say there's a chance his full fighting skills haven't been shown yet. But, Sarah would be more dramatically satisfying, since she has brainwashed herself, and almost turned into Dural...
If the story is just another thing to be saved for the manual, tho? Then it'll probably be whoever the hell the devs like most. One of the original cast has a greater chance, due to their iconicness, but otherwise, it's up in the air. Whatever is needed to give characters an interesting-sounding bio. (I.E. Akira losing to Kage TWICE.)
I do hope, someday, that we'll get a proper story mode for the series. Good or Bad, it would absolutely be entertaining, no doubt.