I can already tell Wonder Trade is going to be in hot water.
I'm pretty sure no trainer in his right mind would put up rare pokemon like Yveltal for randomized trade, without some incentive. Remember Gen 5's GTS? I imagine Wonder trade will be clogged with common pokemon to the point you will always lose if you donate something rare.
Again, some kind of incentive to do so must be put in place. Otherwise I don't see a lot of people getting lucky the way GF envisions 'em to get with rarities...trolls will abuse that mechanic.
...A wise one once told me love was like a pokemon battle. You can fight it, you can run away from it; or you can end up without any balls when the opportunity arises.