I really hope this tournament comes to the rest of us; it's just too much fun theorymonning up cores with Uber Pokemon! Obviously the big dogs in town are Kyurem-W and Reshiram with those ultra power Special Spread attacks. I can forsee Terrakion being everywhere just to deal with these monsters, maybe even Metagross to a lesser extent to help against the Kyurems.
We still have Sun and Hail for auto weather, so I guess sun gets its best chance to shine with Reshiram, Kyurem-W, and lack of Drizzle. Sand has a few things going for it, but I think it will again be used primarily as a disruptive tool rather than a centerpiece set up (although the addition of Iron Head to Excadrill's movepool could mean otherwise...) I can see manual Rain still working when you have excellent inducers and users like Tornadus and Thundurus but I'm pretty sure manual Hail will see next to no use. Gravity gives you those same perfectly accurate blizzards, gives Ground-types a major boost, allows you to fling around a few deadly but inaccurate attacks, and even got a distribution buff with the move tutors.
This the part where I theorymon more than likely terrabad cores I'd like to try in the tournament.
Special Gravity Landorus + (Choice Scarfed) Kyurem-W
As I mentioned earlier, Gravity sounds pretty cool to throw around Blizzards with and what better Pokemon to activate it than a Pokemon that can abuse it without hurting its partner! Sheer Force LO Earth Powers are going to hurt when only Grass-types can switch in. Those Grass-types certainly won't be enjoying the force of unearthly powerful Blizzards from Kyurem-W. The set up is simple, protect or attack with Kyurem-W turn one while Landorus sets up Gravity, then smash everything. The core is kind of slow on its own, so a Choice Scarfed Kyurem could be an option to smash stuff out of the gate. The only downside would be dealing with marginally less accurate blizzards for a turn.
Meinshao + Tailwind Reshiram
Hitmontop and just about every other cool Fake Out user is gone, meaning we're stuck with these rather lackluster ones again. Meinshao is second only to Weavile and Sneasel for the fastest Fake Out users around. Fake Out can buy some time for Reshiram to set up a Tailwind, allowing Reshiram to spam Heatwave while Meinshao Low Kicks any pesky Tyranitars.
Terrakion + Skill Swap Meloetta
Skill Swapping to Terrakion not only restores Terrakion to his former glory of hax king in VGC 2011, but it could also make your opponent reluctant to attack you with Dark-type attacks when you could be carrying Relic Song. From there, it's a simple matter of spamming Rock Slide and Hyper Voice until the opponent's down. What's best is that this core doesn't rely on Skill Swap at all to be effective so it might actually be viable!
Without a doubt the most gimmicky option, but Meloetta might just have enough Special Defense to be viable. Terrakion is a cool cat when Rock Slide deals with Kyurem-B/W and Reshiram and Skill Swap can make him super duper annoying. Meloetta can also throw around paralysis for speed control, or you could just try a Scarfed Terrakion.