Though it seems that this won't end until after someone wins, I'll add my thoughts.
Luxchomp: You know, I've never seen or played against one personally, so I can't really form much of an opinion. But, it DOES seem like it can, and will either win or get close.
Jumpluff and versions of it: Jumpluff is pretty consistent, no matter what anyone says. As long as they act smart and play right, Jumpluff can easily win.
Gyarados: I think that it's not going to win with so many Luxray's around, and if they have a prized Magikarp, or it gets sent to the lost zone... Heh, gg.
Cursegar: It's all depending on the opponent of how it'll go, though it does stand a chance. I give it credit, it is decent.
Dialgachomp: It's good, until you meet one hard fire type. The ability to take out poke-bodies can be handy
Donphan and versions of it: It's good. 60 for 1, it could bring up a fight. Just, hope you don't get matched up with Jumpluff.
Feralistoise or Feraligatr/Blastoise: It can do good, but with Luxray hanging out everywhere, and the fact that it's just so slow... Can't smile much on this deck
Sablelock: This one has hope, very much. Easy donk on some area's, but not so much on others. It does have hope, and has much potential to do good.
Kingdra: Can do very good, just hope people don't tech a fire type in for Prime. I give this one hopes of doing good.
Shuppet: Never faced it, thus cannot give an opinion about it, sorry.
Rogue: I give rogue deck's the best chance of winning. Who knows, something might randomly shoot up and win. With many old good cards coming into close, people are going to want to use them to the max, after all. 'Least one can hope a rogue does well...
I'm sorry if I missed out on any other very popular deck types, as my knowledge is limited, like others. But, I do hope that things go well at Nat's, as who knows what might happen *laughs at thought of Bidoof winning everything*.