Even though I expect (or hope for) a patch I want to clarify some things for the arguments against a patch. Because it is kind of reasonable.
I saw a video with the information on Volcanion, that you get when you go with it to the Press place in Lumiose. It's in the game for sure. There WILL be a patch. It was possible to battle and trade between RSE and FRLG. It was possible between DPP and HGSS. It would be stupid if ORAS weren't compatible with XY
ORAS and XY will be compatible with or without a patch. The patch would simply fix the compatibility issues in a more "seamless" way when they would otherwise get handled in ways that are much more "awkward". As in, every potential compatibility issue that arrises from the new content in ORAS has been handled in parrallel examples of previous other mid-gen additional content.
Kurona and
Cobraroll already explain the majority of this above.
- New forms are recognized aesthetically as old forms and displayed as such, and the stats/types are still given by the ORAS code.
- Mega transformation sequences can be simply a text alert or could still show the animation but with no form change. (EDIT: and show the mega symbol like suggested by tea_and_blues below)
- Pokemon holding new items are restricted by ORAS code from being traded to an XY player.
- similarly if Primal Kyogre and Groundon are using new moves (as they seem to be) then ORAS code can simply prevent ORAS players form bringing Pokemon with those moves (and any other new moves/abilities) into battles with XY players.
Compatibility is totally possible, and these issues have been solved before. It's just that now we play the game with PSS, and these potential issues are extending to a sizable handful of Pokemon rather than just 1 or 2. It makes these solutions seem kind of sloppy and awkward. When you consider how much interaction takes place on PSS and how pretty much every one will be using new megas when the games come out, etc. These "sloppy" solutions are pretty much guaranteed to be experienced by anyone playing XY online and will certainly erk a lot of casual players of the game who now are much more interactive.
I'm pretty sure there would be a patch. All of that unreleased content on XY's code—besides some beta objects I guess—screams "future events" all over. Remember when Mega-Blaziken came around? No way to get in-game. That's what they want to do. They want to release Steam Eruption Volcanion, Hyperspace Hole Hoopa, Thousand Arrows and Thousand Waves Zygarde, and Lati@site in the future similar to how they released Blazikenite and Diancie.
Hidden content is the confusing part. Lati@s mega stones, Diancie, Volcanion, Hoopa, (with their moves) and the Thousands moves are all in XY code yet all the ORAS content is not (including the mega for said XY hidden content, Diancie) You can come up with a lot of reasoning as to why some content is or isn't included but in the end you're guessing so we can't say for sure. And then there's also the small (I think it's small anyways) possibility that some or all of the "hidden" XY content is just beta data that they didn't take out of the games (I think that's really only even possible for Lati@s megas, but idk) However Diancie is already confirmed non-beta, so the fact remains that some content was included and other content wasn't. If there are going to be patches it raises the question: Why not just patch all of it or none of it?
The only reasonable answer I can come up with for that question is this: They are treating event content and ORAS content differently. Patches can make computability less "messy" but a game that requires 10 patches is kind of annoying and a little "messy" in its own way. ORAS requires one patch were as event content would be patched with numerous small patches at each release, which is worth trying to avoid. this still begs the question: If you want to avoid patches why not include the ORAS content in XY too? which is why it's flimsy reasoning. Alternatively you could argue that perhaps all the new ORAS content just wasn't ready in time for XY. Although that is also kind of flimsy reasoning since they control the release date/ plan ahead and all that stuff.
So, like I said in the end we can only guess. But the question: Why not patch all of it or none of it? Is worth considering. Because it does make some of the choices they already made seem a little off if they were planning to patch in new content.
Honestly, I don't even care about the theories of why/what/how I just want a patch because this is 2014. Even if GF doesn't allow for new megastones to be traded to XY, players should at the very least be able to see what they're missing out on. Just the thought of XY not being patched irritates me.
So there is some confusion in the reasoning for how content is or isn't included in XY that makes content patches seem kind of weird. And there is also ways that compatibility can be maintained without a patch. But in the end the reasoning for a patch comes down to exactly what
Chill said. The past solutions to compatibility issues are just outdated. It's 2014 and they've already patched the game twice so why wouldn't they do it again? Especially given the magnitude of possible "awkward" and "sloppy" interaction that will ensue if they stick to these outdated solutions?
And then the attitude I imagine
Kurona and
Cobraroll are taking (though I can't speak for them directly) is more like: The fact that they didn't patch all hidden content seems off and I wouldn't expect too much out of Troll Freak. There will be compatibility without the patch so why would we expect them to?
There is no definitive proof one way or the other. You can really only speculate based on those two different attitudes about how Nintendo will handle it. This isn't really meant to be a strong argument for either case, just an explanation of everything we can figure out/ guess at so that we don't have to repeat the same arguments over and over.
(now I'll turn in this essay...)