I'm pretty sure Lucario, who is a Gen4 Pokémon, appeared in many Gen3 sets, including a Pokepark promo card. So, Magearna could appear in this set.
Lucario only appeared as promo cards and special decks. And only in Japan. We never got a Lucario card before DP in the West...
EDIT: ninja'd.
btw, to the user asking for a gardevoir spirit link: we already have it.
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Another Mega Gardevoir??? This would be nicer if we didn't already got a FA in Pokekyun that features a shiny Mega Gardevoir.
Also, I'm thinking the same as other people posted: naming the set Cruel Traitor seems like they're spoiling the movie as either Mega Gardevoir or its trainer (who can be seen on the side of the protagonists on the movie poster) are actually sided with the villain. When you consider that and the fact Volcanion will be some sort of an ally in the movie, yet it's separated from Mega Gardevoir for its own subset, makes even more sense.
If that's not the case at all, then why not feature shiny Mega Gengar instead (which got a promo in Japan that we never got) or one of the other Megas on the poster?
Speaking of which, if we're supposed to base our guesses according to the pokémon movie, I hope each subset set has 3 megas instead of 2 (as there are still 14 megas unreleased and we're getting close to the end of the XY era) and they avoid repeating Megas (except Mega Gardevoir of course). From that list, Mega Salamence, Mega Beedrill, Mega Steelix, Mega Banette and Mega Sableye haven't been released yet and together with Mega Gardevoir, they make the 6 megas I'm hoping for.
I'm also inclined to guess the mythical cards we've been getting in the boxes throughout the year come from that special set. But if that's true, I wonder what will be the fate of the rest of the cards when they come to the west (probably stuffed inside a main set or released all in a box as promos like what happened to Legendary Shine Collection)... This one has secret cards though.
I would have also appretiated if they had made a regular Zygarde Perfect forme card for that promo blister. Only having 2 of the forms sounds lame to me...
Finally, as some people mentioned Mega Camerupt and Sharpedo, I think they forgot about them... At first I thought they were going to be on Double Crisis after being absent from Primal Clash. Then, I thought they would be promos... They just never happened.