Had a idea for the food Yveltal like and it would be chocolate. She would like it since it can make a person feel a little happy and since she has been hated and feared she would take any kind of feeling of happiness she could get. Now that she is with the MC she now has access to ALOT more chocolate and any food with chocolate. Since it was said that as she gets more life energy she would get plumper, her eating ALOT of chocolate at a time can increase her weight with it. She would also think there is more of here for the MC to love and love it.
Her dream could be that she dreams she is Charon the ferrywoman of the river styx taking souls to the land of the dead.
For Xerneas food with her being chaotic her food would also be chaotic where it would be weird food combinations she would be craving to have combined like for example: pickles and peanut butter sandwich with wipe cream frosting.
Her dream could be something like the game "Goat Simulator" that is chaotic with things that make no sense but fits. When the MC goes to see the dream they find Darkrai in the room looking at Xeneas dream and taking notes on how to make dreams more chaotic from her.
It would be funny if since Yveltal is not as evil as Darkrai thought she would be surprised that she likes the chaotic side of Xerneas even though its a fairy type.