"Xerneas BREAK, and Yveltal BREAK"
All aboard the Dark and fairy type hype train.
In all seriousness, this yveltal could either completely change the dark archetype and create an even stronger dark deck, or it could be Luiga BREAK levels of meh and change nothing. As for Xerneas BREAK, hopefully this BREAK introduces a decent attack that can allow Xerneas to better fit in the Gardevoir archetype deck, or even provide an ability that will by pass the loss of Aromatisse.
I'm also willing to bet that the new Gardevoir will be a fairy type that will somehow enhance the "Brilliant Arrow" Gardevoir in some way. At least, the Radiant Collection Gardevoir seems to suggest that if someone wants to run three M-Gardevoir, then they want to run x1 Primal Clash , x1 Generations, and x1 Steam Seige (imposing a 2-1 split).