An idea for Xerneas is that she has a massive pregnancy fetish since she does make more life and once she sees how much the MC filled her up and just imagines the number of eggs and how big she will get she could drool at the thought and look forward to seeing Yveltal massively pregnant and unable to fly. Once the MC brings her to the ship she can comment that she has come to her dream paradise of being surrounded by massive pregnant belly's all the time now. Zygarde would sense that Xereas is making less life energy since she is pregnant. With her around the number of eggs each legend can make could increase from a dozen to a dozen and half or two dozen and just be mentioned in the egg laying scene otherwise everything else would be the same unless chainsaw wants to put in a image of a legend with a extremely large belly where it is more than twice the size of their body and would support them entirely in the air. When the MC finds both Xerneas and Yveltal they would be just coming out of their tree and cocoon forms so don't need to worry about them changing mid journey later and both would warn the MC "You don't need to worry about finding Zygarde. What you need to worry about is her finding you instead."
I had thought of a name of a group the dragon types make for their weakness, DPC (Dragon Pleasure Club) they would get together to find how to maximize their pleasure. They would think they have the superior pussy for pleasure since usually dragons look down on other types and having a dragons pride would have them in competition with the AWC.
That can lead to a side scene on the boat involving Latias and Latios to decide which type is better for pleasure for the MC. They would decide that neither is better but they themselves are better than both groups since they can feel intense pleasure from front and back at same time and send them to levels of pleasure that the others cant reach and make both groups jealous of them even though they are the smallest dragon type and not the strongest psychic type.