I've come up with two possible non-planeswalker duel decks sets that make some sense but could be hard to pull off.
Every color has been used at least twice, except Blue and White (which happen to be ally colors, unlike past duel decks). I'm assuming WOTC goes for a semi-tribal/rival approach like Divine vs. Demonic, and although merfolk comes to mind for blue, I simply can't think of a rival. However, I can think of an appropriate theme that could be a lot of fun: Swords vs. Sorcery. By that I mean a White knight deck against a Blue wizards deck. Of course blue will make use of counter spells and a lot of sorcery spells, but I wouldn't want it to be a milling deck, and at the same time not a complete counterspell deck. I'm thinking taking control of the best creature, looking at player's hand, drawing cards (Riptide Director immediately comes to mind), some morph; basically the type of thing you'd expect from blue. A white knight deck doesn't require much imagination, I think most people have a pretty good idea of what to expect from that. The only problem is that wizards can be pretty outmatched, so some "return target permanent" should do the trick.
If blue/white won't be used, then that makes room for one rivalry that makes absolute perfect sense, the classic fantasy battle of Knights vs. Dragons. The first problem that comes up is that dragons have Flying and knights generally don't. There's ways around that and I'm sure that two decks can be made to play amazingly.