Why is everyone counting on a third version of XY to be released? We were all expecting a "Gray" version to be released but instead we got Black 2 & White 2. I'm pretty sure that GF considered a third version to be an outdated concept and they switched to the concept of sequels. My prediction is that there will be XZ & YZ versions. You guys can all call me the "prophet" in a year or two down the road, folks (or not, if I'm wrong
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BW2... was a surprise. Though it really shouldn't have been, since the hints were there. Namely the moves Ice Burn and Freeze Shock, which existed in BW but had no user. Both were the same in different spectrums (140 power, 90 acc, two turn charge, same type, 30% rate, same PP, one physical, the other special). And by description, one was Kyurem mixed with Reshiram, and the other with Zekrom. No one saw the fusions showcased in two different versions coming because it was never done before. Hence, surprise.
Looking at XY now, there are two moves that have the similar status. Thousand Arrows, Thousand Waves. Both Ground, physical, same power, same PP, same accuracy. Both have no user. But clearly, they'll belong to Zygarde, because what else is there? It already has Land's Wrath, which has the exact same detail as the two Thousand moves (power, acc, PP, target, spectrum, etc). It's not hard to think that that move would change into the two Thousand moves, just like how Glaciate turned into Ice Burn/Freeze Shock. Thousand Arrows targets Flying types. So, if Zygarde was to target a Flying type, what would that be? Yveltal. And if one of the two "duo" moves hits Yveltal, the other is bound to hit Xerneas, even though it doesn't sound like it's designed to.
So there are 2 moves for Zygarde that haven't been revealed yet, and they target Xerneas/Yveltal. Way too similar to the time of BW transitioning into BW2. So yes, a dual "third" game seems possible, where instead of fusions, there'd be Megas or something for Zygarde. Go from axes (X, Y) to planes (XZ, YZ). But, I don't know if BW2 was enough of a success (marketing wise) to warrant a repeat. Making two similar yet different games (again, after the initial two) need to have a certain amount of benefit to try again. Like Pokemon Movie 14 (a & b). Some would watch 14a. Some would watch 14b. Some would watch both. But the benefit from the A-intersect-B has to be high enough to cover the effort of making both A and B. How many people got both B2 and W2? Did enough people get both to make BW2 better than just G in terms of returns per cost? If yes, then okay. XZ/YZ (or whatever) are likely. Else, I doubt it. A lot.
Right now, I'm more leaning towards a single third than a dual. If dual, then Generation VI would have X/Y, OR/AS, XZ/YZ. Every stage, people would get one of the two, and not both. Such as get Y, then AS, then XZ. Have a buddy get the other 3 or something. Most people would do something like that, which really wouldn't be efficient in terms of returns per cost. While if there was a single third, then most people would get that. Think, for every person (or just about) who got D or P, would get Pt. But, for every person (or just about) who got B or W, they would get either B2 or W2 and not both. More games made, same number of purchases. The dual-games plan = dud. Extra effort, no extra gains.
Unfortunately, we're going to have to wait until ORAS is over to find out for sure. Which will be... a year? In the meantime... make harmless bets with friends or something.