Zygarde will probably get a new form and be more powerful / useful if they make a Z version or X/y2 (like Giratina and Kyurem)
as for typing, I didn't know about Fairy type or anything when I got the game!
I hope that's the case, Zygarde just plain out sucks now. It would make sense for Zygarde to get an upgrade and I'm also hoping to be able to get it's shiny form ^^
Anyway I agree with Yveltal being way more powerful, just look at their signature moves! Yveltal has Oblivion wing which has 80 power but also heals 75% of damage given while Xerneas has Geomancy which has to charge one turn before getting the stat boosts.