Shallan and Szeth are too obvious for going evil, and Shallan has already made progress away from being a thief/probably justified murderer.
How much detail do we have on the "one of them will destroy us" bit? Could it be an accident? Like 'oops, I accidentally unleashed the ancient evil through a series of totally logical/well meaning events' type thing; or is it set in stone that the one will be a full on intentional harbinger of doom type thing? To me, the first would be more interesting (especially if it is Szeth and it is the finally straw for him and his magic rock of ownership).
As for shipping, all of the younger guys seem like pretty good men, even if Renarin is a little under developed at the moment, so I would be happy to see Shallan interested in any of them. The contrast between honorable Kaladin and somewhat less so Shallan would be interesting; while both Shallan and Aldoin are both working on the whole honor thing; brother number two maybe this will give him some character or they will be an action girl/nonaction guy duo. Shallan will get action girled next book, I'm almost certain of this.