Well, the tricky thing about pointing out dark horse teams is that they wouldn't be dark horse teams if people noticed them. That and lots of people like to rebuild at the last minute.
But if you want to try and spot these teams, check the smaller and less-publicized regionals. Look for the rookie teams, and anyone with an unconventional strategy or design. Don't underestimate them, because (as proved last year) they can be just as good as or better than anybody else.
(Yeah, I'd nominate my team as last year's dark horse candidate - all-girl rookie team, purely defensive robot, did our best work at a regional with 24 teams attending, ended up being surprisingly effective mostly because nobody saw us coming. Not this year, though.)