Jax vs Fiora
Jax. No question. Fiora would kill Jax if she actually beat him on the bridge. Moreover, Fiora's clearly in need of a goal for her story to advance properly, and beating the upstart hobo that managed to defeat her in single combat would definitely push her to get stronger. Jax is meant to be 'the champ', a guy who fights so well and so hard he had to start handicapping himself just to get a real challenge.
Xerath vs Aurelion Sol
Xerath's been stated previously by riot to be capable of literal omnipotence (along with Syndra), so he could beat Aurelion Sol. But it's unlikely they'll ever fight, because Xerath's story is tied to Shurima, not Targon. Moreover, I'm not sure how Xerath would attain the full omnipotence that we've been told he has, or if Aurelion Sol would let him to begin with. Minor things, ya know. But I'll give it to Xerath for now.
Xin vs Yi is much more questionable, because there's no real story reason (that i know of) that one or the other would/could win.