I've seen some people proposing things about Yveltal and Xerneas, and that gave me some ideas :3
About Xerneas, her having a huge pregnancy fetish is of course a wonderful idea ! But about Yveltal... What if she is even more addicted to that than Xerneas ? Xerneas is used to be pregnant, she just want to be hyperpreg and be surrounded by hyperpreg people and when she met the MC that'll happen, but Yveltal never was pregnant. I propose that she crave to be extremly pregnant to the point of not being even mobile, but with her powers the only ones who didn't fear her are people looking to use her powers. So that may be a duality between two huge lover of hyper pregnancy with Xerneas used to making love and be impregnated, even if she never was as pregnant that when the MC impregnate her, and Yveltal being a virgin who only can dream of that. Maybe making so that Yveltal and Xerneas are very close, that they love each others but because Zygarde force them to respect their roles and that the godess of death must not bring life in Zygarde's mind, Xerneas cannot help her and when the MC impregnate Yveltal Xerneas is very glad he did so ^^ I really see them as close friends / lovers who live an impossible relation, that would be so cool in my sens ! And Zygarde, maybe her too love hyperpreg but she didn't realised it, she discover it when she discover the MC, that she feel herself impregnated and imaginate herself with a humongous belly ? And that after accepted that Yveltal can get pregnant she feel guilty and want to make the two legendaries the happiest because of this feeling, but now the two have no hard feelings against her because she's their friends and was just doing her job.
I would see this relation of trio as opposite as the Groudon - Kyogre - Rayquaza, where Groudon and Kyogre hate each other's and Rayquaza is kinda annoyed by them and make them learn their place. In the Yveltal - Xerneas - Zygarde trio I would see it as Yveltal and Xearneas cheer up Zygarde and they love each other's ^^