I cannot really see them being used, unless they work like the Level X Cards. And, if that is the case, for the time being, they really CAN'T be used, because that would mean some Pokemon would be hitting up to Stage THREES!
But, of course, I am thinking more negative than positive, here. I was not happy with the news of Mega Evolutions, really for the fact that we now have Evolutions of...stage two Pokemon [exp: The starters, Ampharos, Garchomp]
But now, let me not be Bias...or...the opposite of Bias...ERh...Yeah, Bias.
Idea number 1:
From what it looks like, you need an item in order to evolve them in the games. So, that means that they would need to have an Item in order to evolve them in the Card game, Like the Rare Candy. They would most likely have much higher HP, maybe even surpassing Wailord [unless they will replace the EX]. However, they would have to evolve from that item card.
Idea number 2:
They will not replace the EX Pokemon Cards...but they WILL be the EX cards. Like, back in the day, you could evolve into Primes or EXs.
Idea number 3:
They would be like...some card where you can only have on of, but they would be Basics. [Like the Star Cards [Shiny...Star Cards, not just Rare Pokemon Cards]] They would be more playable if it were like this, but...you would only be able to have one.
All I know...it will make the game more crazy! If they do it right, it is a good crazy...
But if they do it poorly, there will be an angry Eevee in the room...but, considering how the EXs are working out...I'll probably go and roll with it. I mean, I find the card game better than Competitive VCG!! That's SAYING Something!
But...what I am looking forward to...is SYLVEON! Oh wait...I'm getting off topic.
~Misty