Yveltal would love the DARKEST of Dark Chocolate!
I can see her in the dream wondering why her boat is sitting lower in the water than it used to, she's chubbing up in her dreams too.
Xerneas probably would want to experience as many things in life, that would likely mean mixing random food together.
The dream fits too, she'd come up with the most bizarre nonsense ever, maybe like the Meet the Pyro video?