Just going to give one last reply to all of those that quoted me:
Yeah, they created these forms but decided to push them into the new generation games with no reason... Like "Hey guys, look at these cool new Kyurem forms, they are Black Kyurem and White Kyurem, but guess what? You will see them in XY". Nonsense, just like this Zygarde case. Ok, let's pretend Zygarde 10% can transform into Zygarde 50%. How Zygarde 50% can transform into its Perfect Form if it has another ability (Aura Break)?
Do you even have proof they actually planned and were going to do Pokémon Z? Because the way I see it, it was always speculation from us fans, based on the sole fact that we expected this from GF. Just because there’s no Pokémon Z to serve as this sequel game, doesn’t mean that Sun and Moon can’t do the same, and perhaps better, while being a new Gen on the same road. Gold and Silver did the exact same thing with being a sequel to Red and Blue, while being a new Gen at the same time. It’s not unreasonable to think Zygarde may have been a part of Sun and Moon the entire time since its debut in X and Y. After all, there have been a ton of references hinting to a new region, which made some people, including me, think that there was a case of Gen 6.5 happening, which we now know isn't going to happen, and instead we get Sun and Moon in its place. Really, what sounds better to you?
1. Pokémon Z with a few new improvements and additions with a Zygarde focus?
or
2. Pokémon Sun and Moon, a new gen with new mons, along with most of the introduced features of X and Y plus a focus on Zygarde?
I think I would go for Option 2, and the chances for that happening are pretty high at the moment. Sure, we don't know how Zygarde is yet going to be involved. Maybe it will be awkward, maybe it will not be....we just don't know yet. Its only known that Zygarde is in Sun and Moon in some way or form. That has been officially confirmed now. I myself am willing to bet that this was always the plan from the very start.
As for your question regarding Zygarde’s transformation power/ability, I speculate that Zygarde 10% forme can only transform either into Zygarde 50% Forme or its complete forme depending on its current health state. That way, Zygarde 50% Forme keeps its Aura Break ability. Or this is not the case and Zygarde 50% Forme gets Zygarde 10% Forme’s ability. If the latter, perhaps Aura Break was just a filler ability. Also to note is that Zygarde Complete Forme has yet a unknown ability, and its likely not Aura Break, otherwise we would have seen it in the trailer just now.
Now hold on a bit. It's not yet a fact that there was no plans for Pokémon Z, the evidence just points towards that possibility. We don't yet know for sure what their plans for Zygarde are, all we know is that they hinted towards a connection with Kalos and Xerneas/Yveltal and now they're showing up in XY.
And yes, I would agree that the execution of this all is a mess regardless of whether it was planned or not. Or at least that's how it looks right now. XY left a ton of loose ends specific to Kalos that I was hoping would be explored in a follow-up game of some sorts (I was hoping XY2, but Z would've been acceptable as well). Yet we're moving onto a new generation in a new region and we're seeing some of those problems carry over in a potentially unrelated region. There's a chance that this could all work out somehow and SM will finish what 6th gen started, but I'm doubtful that they can pull it off without it feeling awkward.
Be honest. If there was Pokémon Z, it would have been released ages ago, or at least been known by that time, no? GF never acknowledged its existence, other than in a interview that they didn't want to go forward into a predictable manner with a Pokémon Z. By then, that alone should have been the sole clue that Pokémon Z was never going to come. However, it were the fans, including me, that held on to the idea of a Pokémon Z and possibly a Gen 6.5 still coming in a similar way like how Black and White did it, but it never did. Instead, we got Sun and Moon in its place.
Sun and Moon were likely always planned from the beginning along with X and Y. I mean, why would the X and Y games just bluntly hint about a brand new far off region, talking about the War between Kalos and another region, the mention of a Moon Dial, and even give us a item depicting a Legendary that is revered as some protector in a new region? I’m pretty sure that hasn’t been done at the very last moment of development regarding the X and Y games because they decided to scrap Pokémon Z at the last minute.
If people can see that Sun and Moon could still be the sequel game to X and Y, in similar veins to how Gold and Silver were sequels to Red and Blue but were also a new gen at the same time, it becomes pretty reasonable, and becomes a way better set of games than what a singular game of a possible Pokémon Z would have done. What is the reason for doubt, for at the moment right now it’s way too early to even judge Sun and Moon based on the current info we have. We only know that Zygarde and its forms are in, so now the connection/speculation of Sun and Moon being the sequel games to X and Y is now made stronger.
Yes, there is. It doesn't matter if they didn't actually planned Z. It should have happened, period. Either Z or something like BW2, they should have made a game focusing on Zygarde as the 3rd mascot (now it will forever be known as the mascot legendary that it's not a mascot legendary), properly introduce its formes, give it a key role in the game's plot (which will never happen in Sun & Moon cause those games already have their own legendaries to worry about), explain its actual relation with Xerneas and Yveltal, gives us AZ's Floette and resolve all the other mysteries in XY. There's a lot of things in XY that could have been more explored with a third game based on Kalos.
It's also quite messy that, with Zygarde being the guardian of Kalos and the new forms appearing in the last Kalos story arc of the anime, they make the new forms inaccessible in its native region's games and instead debut them in the first pair of games of the next gen, in a new region that has nothing to do with Kalos (that we know so far)...
You can say the same with Pokémon Grey then. Should have happened too, but didn't. Instead we got sequels to Black and White, and we didn’t even get the Original Dragon on top of that, but only a pair of intermediate forms for Kyurem. Based on that, Black and White are still unfinished, technically speaking.
Again, Zygarde's formes and its concept were long done before this, and were likely always a part of Sun and Moon as it was with X and Y. Again, there has been a ton of references in X and Y about a new region, tales about a Moon Dial, and more. We even got a item relating to a Legendary that came from a "far off" region. That region is very likely the Alola region within the Sun and Moon games, because it’s obvious that Alola isn’t just next door to Kalos. Sun and Moon were always planned to be a sequel of sorts similar in vein to Gold and Silver being Gen 2 and sequels to Gen 1's Pokémon Red and Blue. Only this time, there's likely going to be a Post-Game story arc or something else that ties Zygarde to the Sun and Moon games and explain its Complete Forme.
There was never going to be a Pokémon Z, because people would have likely complained there isn't much to do in such a game, if there was one to begin with. GF wanted to make surprises to their fans, and they did so successfully aside the messy marketing with Zygarde's formes. Just explaining about Zygarde and it formes requires a mere sequel? And then what? Catch the same old Pokémon again? Again we barely know anything about Kalos's complete lore. For all we know, it can still link back to Kalos. Really, just because Zygarde's formes were introduced in Sun and Moon suddenly means there's no connection to Kalos?
These are all early assumptions to make. The games aren't going to be out for another few more months until late November. I'm sure we will hear more as the release date comes closer.