IF they were going to do a Sasuke deck as water, I would have guessed a Sasuke-Water deck and Kakashi-Fire deck. But since Sasuke seems to be fully Fire (and even lightning) now, I don't see that happening.
In this case, I'd say the Fire deck has got to be Sasuke(maybe Uchiha, but definitely Sasuke as the main card). I love Madara as much as the next guy, but Sasuke/Kakashi is a safer choice, marketing-wise, than Madara or Tobi/Obito.
As for Water, Seven Swordsmans seem like an option to me; maybe even new Water/Void editions will be printed. I don't think they're going to go the Kisame route again, so I'm not sure on the title water character.
It'd be interesting to see the sales figures on the previous starter decks, so we could see which characters sold starters the best. If Kisame was a huge hit, then they may go back to him again, but if his starters were signficantly lower sales than Gaara or previous Naruto/Sasuke starters in QFP, then they may play it safe and choose a more main-stream Naruto character.