I don't think we should use the version of the Duel mechanic that requires discard. During the GDS2, Discard-Duel had the problem that it often caused people who won Duels to lose the game because they discarded high-impact cards. Mark Rosewater said he believes the mechanic is unsalvagable; we have to remember that's how bad the problem was.
It's been proposed that Madness can solve the problems of this mechanic, but I disagree.
First of all, how reliable is that as a solution? How many cards with Madness are going to be in people's decks? People will only occasionally have Madness cards to discard. We can't just accept that people will have bad experiences whenever they don't draw a Madness card to go with their Duel card.
Second of all, the Madness cards are going to be virtually stamped with "please discard me with top priority." I think that the beauty of Duel is that it turns all of your Magic cards into potential Dueling pieces, because every card has some converted mana cost value. Some people may disagree, but I personally think it sucks when there's a clear best card for discard and most of the cards in your hand are not real choices for discarding in the Duel. Duel is fun because you look at each card in your hand, contemplating which card is going to give you what chance of winning the Duel, and then make an actual choice. We should not be distracted with secondary goals like getting kicks out of combos (such as with Madness); the first priority should be to make Duel itself work as a strategic mini-game.
Thirdly, your opponent can discard cards with Madness to your Duel spell, and it would feel really bad when that happens. What I imagine will happen is that players will quickly decide they should only cast Duel spells when the opponent is tapped out, or when the caster of the Duel spell also has a Madness card of his/her own. I don't think that's ideal; people should just Duel whenever they feel like their cards can win the Duel.
Finally, I feel it's wrong to do any mechanic just for the sake of comboing with another mechanic. While it's best if the mechanics play well with each other, that's not enough; each mechanic should play at least decently on their own. Discard-Duel has a bad effect on game play on its own, while Madness is one of the worse mechanics in Magic IMO and has no real connection to the set theme apart from Duel interactions. We could just use a fixed Duel instead (Permanently-reveal Duel, unless a problem is discovered with that).