Something set in Planescape could be ridiculously mega awesome. It would also be an excellent setting if you did want the 'modern teenagers get sucked from Earth into fantasy world' thing; then it could revolve around the characters trying to find the right portal back home (from Sigil). Alternatively, the characters could be youngsters drawn from a typical, medieval fantasy earth (like Toril), and you could have the same sort of dynamic but with cooler characters (who could also be of traditional fantastic races like elves and such).
I'm picturing something a little Men in Black-ish, with the clueless main character(s) winding up in a world that seems relatively normal at first glance, but then starts to pervert the character's expectations at every turn; i.e. clueless Joe is walking down the street and asks a robed figure where he is, and when he sees the figure's face there's a 'what-the-Shar-is-that!' (a dabus or something) moment.