It's kinda hard to answer seeing as we don't know what the rotation will be. I'm guessing XY Base, Flash Fire, Phantom Forces, Furious Fists will all rotate. There is a chance that Roaring Skies will go as well. If Roaring Skies goes Shaymin goes, and the whole metagame changes and will be completely unpredictable. If Roaring skies stays we will still be loosing things like Battle compressor, Blacksmith, Korrina, Dimension Vallery, Fighting Stadium, Seismitoad and other cards that are Staple in current meta decks.
Fire, Fighting, Psychic decks will all take a massive hit potentially. So the chances are those type decks won't remain as powerful as they are now. Good chance that Vespiquen or anything similar that relies on Battle Compressors will get screwed. Although overall Grass will likely remain strong. Water will be loosing the mighty Siesmitoad so that new fancy Aquabox deck that everyone loves might not be sticking around for too long.
But of course we don't know yet wether we will be getting a reprint set like Japan does, and if we do will it make them standard legal or will it just be a way for Expanded players to stock up on stuff like the Darkrai VS Mewtwo deck box that came out but didn't count as a reprint.
Of course we don't know exactly what will be in XY 11 for us in terms of reprints, so it's possible that something might get a reprint in a japanese theme deck or other product then they have to reprint it in XY 11 so it's still in format for everyone world wide.
The short answer is nobody knows, probably not even the people at pokemon who are in charge of this decision know 100% what decks will and won't end up looking strong.