Oh, typical of my luck: The reveal happens on the first of four consecutive days for me, where I'll be extremely busy from morning 'till midnight, with limited Internet access and time to get on my phone. I saw the reveal earlier today, but was unable to read properly into it, or make replies. Because I'm silly like that, I've logged on after midnight to make a reply here, although I really should have gone to bed. Oh well.
Thinking man that I am, though, I've formed a sort of opinion on the news. Ahem:
What. The. Flip.
First of all, one big BOOO! to Nintendo's marketing department. Releasing such news via a magazine notorious for leaking long before release is simply plain idiotic. Every Pokémon fan in the world with regular access to even ONE Pokémon fansite has heard the news by now. Of course, it could be that they planned for it to go viral, but come on... There are so many official channels to release this through properly, but no. There HAS to be some shenanigans with a Japan-only magazine whose effective release date they have no control over. Way to go...
Also, I'm registering a notable lack of a game announcement here. Magiana is presented as movie news, even though we're overdue for a core series Pokémon game. Pokémon Z, where are you?
OK, thinking it through again, I'm not that confused. There IS presedence for a next-generation Pokémon to be revealed before the release of the last game of its generation: Munchlax was revealed in May 2004, Emerald released in Japan in September that same year. As it has been said, Magiana is only revealed in context of an upcoming movie, NOT in the context of an upcoming game. The difference is noteworthy. This, among other things, leads me to believe Magiana is a Gen VII Pokémon. Also, for what it's worth, Marill, Snubbull and Donphan were featured in the first movie, which came out in Japan two months before Yellow did.
It could be that Magiana will feature in Z somehow. Bonsly featured in XD, as we all know, as a temporary capture with an extremely limited movepool and no method of evolution. Then again, Bonsly was, ah, inconsequential. It was just another Gen IV Pokémon, with an evolutionary relative from a previous generation. It lacks any sort of distinction in later generations, nothing that makes it stick out. They spoiled nothing by putting it in XD.Magiana has no known evolutionary relative (although people are quick to point to Klefki), and has a design too elaborate for me to think it's the next-gen equivalent of, say, Graveler or Cacturne. It could be the next generation's equivalent of Lucario or something, but then I don't think they'd put a half-assed version of it in Gen VI (and by Gen VII standards, a Gen VI "port" would be pretty half-assed, lacking important stuff like signature moves or abilities). And again, Munchlax didn't feature in Emerald.
As for the possibility of Magiana being a Gen VI Pokémon, I could buy it if it weren't for the fact that Volcanion exists. If Game Freak had planned to include more Pokémon in Gen VI than were possible to acquire in the base games, they wouldn't have bothered to put Hoopa, Diancie and Volcanion there for the world to find. They'd have put in Diancie and maybe Hoopa in XY, then made Volcanion ORAS or Z exclusive. Or just have all three be included via patches. They wouldn't have done it halfway by including, say, 4 out of 5 event legendaries in the games. What would be the point of including Diancie, Hoopa, Volcanion and Floette-E, more than half of which would be released more than one game post-XY, if they had planned to include a fifth event legendary in the same generation? Conversely, what's the point in making the fifth event legendary accessible in a completely different way than the four others? The games are planned far enough in advance that they'd know about Magiana well before XY shipped. I recall somebody saying in an interview that the process of making XY started some point after HGSS was shipped. I'm willing to guess they have had the entirety of Gen VII planned out since long before ORAS came out.
There is, of course, a remote chance that Game Freak has decided to screw all reason and inexplicably made Gen VI Pokémon not compatible with the Gen VI games. They did break inter-game compatibility with the signature moves of Hoenn's weather trio in ORAS, and the Megas are famously incompatible. Then again, Deoxys-A/D/S were incompatible with Ruby and Sapphire, the Griseous Orb is incompatible with DP, you can't trade a Kyurem-B or -W to BW1, and so on. Incompatbility is nothing new, actually. It's just that they've never made entire Pokémon completely incompatible with the base games. They tend to put everything that needs to be compatible in with the code of every game, then release the actual content later.
So yeah, TL;DR, Magiana remains a Gen VII movie-exclusive in my book for the time being. The next generation's equivalent of Marill/Kecleon/Munchlax/Zoroark/Mega Mewtwo. It might feature to some extent in Z, but I'd be surprised if it were fully catchable, and doubly surprised if it wasn't presented as a Gen VII Pokémon when that time ineviably comes around. Nobody counts Bonsly as a Gen III 'mon, I think nobody will count Magiana as a Gen VI 'mon either.