Every year it is the same...
The first year I attended Adepticon followed the release of the 4th edition Tyranid codex and there was so so many Big Bug armies there - I'm not lying if I said they accounted for anywhere from 40-60 percent of the total armies. The year following the release of the 5th edition Space Wolf codex saw lots and lots of Space Wolf armies in Chicago - same thing with the 5th edition Imperial Guard codex. TriFalcon-Eldrad-Avatar-Harlequin spam was the big army the last year for 4th edition... They had an invitational tournament that year and I'd say eldar accounted for at least 70 percent of the armies in that event.
It is a constant that whatever is the flavor of the year will be the most popular army. There is something about Grey Knights though that I predict will throw the proverbial monkey wrench into the works... Psykout Grenades - they are the bane of Grey Knights and ironically enough it's kind of fitting in my opinion. When the fourth turn rolls around it's hard to avoid melee any longer as you're usually fighting for objectives... on the other hand SW versus SW is not such a big deal as they all swing at the same initiative for the most part (excluding characters and power fists). This year you have to factor in the psykout grenade and I've seen Strike squads maul Paladins - it can and does happen. Unless you've played lots of matches versus other Grey Knight armies with your own Grey Knight army this little piece of wargear can throw you for a big loop. In my mind most people who'll be bringing Grey Knights won't be properly prepared for the mirror matches - those that are will profit at the tables.
Sure psykotropic and rad grenades are just as deadly if not more so but you can't take them in every squad like the psykout grenade. Smart players will wisely avoid those units like the plague! It is what it is.
It seems like the two big Internet meta lists for Grey Knights are Draigowing and Purifier spam. I give the nod to the former for a better overall performance as little five man squads of Purifiers aren't really all that scary versus other armies that have significant dedicated melee units. If it were me playing my Draigowing versus a Purifier Spam army most likely I'd reserve my psyflemen and bring them in firing at the opponent's psyflemen. You cripple a couple and suddenly the game can swing. Versus a Vindicare assassin it's easy - simply outrange him with your psyflemen and keep popping him until he fails a save... And all it takes is one failed save then you push forward. The Coteaz henchmen list is fairly popular but I can't see those fragile units going toe to toe with Grey Knights. Sure there are the much feared Death Cult assassins and rightly so but a smart player will cripple their transport first and foremost then mow them down with stormbolters. There is also the Mordrak list but it's not as popular as the other three.
So I'm thinking this year probably some people relatively unknown on the Internet will win the big events at Adepticon simply because they are better prepared for the mirror matches and I think it's going to be quite interesting indeed.