I've been considering things, and I think someone has already suggested such a thing, but in the vein of Sylveon being thought to having a connection to the bond mechanic, I also got the idea that Xerneas and Yveltal are also connected to it, being that they represent two possible ends of it. I got the idea that Sylveon is a sort of purified, optimized pokemon. This also involved considering DNA and that somehow the bond mechanic makes IVs and EVs moot and that with effort, any pokemon can be made to reach the perfection of it's attributes. Sylveon is the result of doing this to Eevee. Yes, I don't like the competitive circuit, but I want to say right now, I did not do this to cheese them off. I was seeking a way to relate DNA to the supposed bond mechanic, although we don't know exactly what that is yet. It made sense to me, although what would the point of even having IVs and EVs be then? Either they would have to rewrite the entire system, or this is a mechanic to supplement them. Pokemon would still occur in the wild as having different stat values even for similar leveled pokes, but it's up to you to train them to have otherwise. It would still make them unique. EVs would be harder to justify unless they make it more transparent that certain pokes raise certain stats. I consider the way you train to be part of how one "bonds" with their pokemon, so what aspect you train could still be considered to be part of one's bond with their pokemon.
Anyway, Xerneas represents the above result, to reach the peak of your ability through this bond process. What is Yveltal, then? The corruption. Something got out of whack or it's a Frustration-esque sort of thing. To separate it from the happiness mechanic, though, I'm going to say it's more an imbalanced distribution of stats. Order and Chaos. If this is true, then there's another side to the bond mechanic and another form Eevee can take from it. (What it would be, I didn't consider, though. Poison maybe?) Now that I look at this, I see that it's kind of one-sided. The pure side would be more favorable than just concentrating on one or few things. (although that has it's virtues too) It's also possible the bond mechanic can INHIBIT abilities. If you neglect something, it starts to lose it's ability in that stat. I don't know if Yveltal seems like the kind that sacrificed some abilities to gain power in others, though. It definitely doesn't seem like something that would inhibit all abilities. This is just a theory, though. I don't know if it is true. I also failed to consider a middle of the road scenario, if there is one. If I added the above together, I would end up with making pokes have equal value in all stats. A combination of enhancement and inhibition.