Yveltal represents destruction, which is a necessary component to life, that is Xerneus' symbolism. Both are needed to balance nature in way that life can flourish. Think algae blooms. Create too much algae, you kill all of the organisms in beneath the water's surface. When creation isn't moderated, neither is destruction. For that reason, a little destruction of algae would allow for more aquatic life to live more satisfying lives. Imagine if death/destruction didn't exist? Things would be chaotic.
With that said, I could see a third, maybe a fourth legendary that works to either regulate/mediate these two, or conversely, aspire to disrupt the balance.
I like the idea of Z, but there is something to be said for XX and XY chromosomes. A male and female legendary, similar to latios/latias in that respect. Though, GF might just feature these two in this game or a sequel for that reason. Though, I would like an XY and XX to represent the motifs I just listed the regulator(s) and/or distruptor(s).