For the Ultra beasts I had read that they are from different dimensions but should all share common attributes so it could be that when the MC encounters each and inflates with his cum they could immediately give milk to taste that would be uniqe to each depending on type and how they look before they get sent back home due to the MC. Examples: Blacephalon could taste like "explosive cotton candy", Naganadel could be an alchole taste. It could also go that the MC gets sucked into a ultra wormhole and meets Ultra beasts and have to meet their demands to be able to pass to eventually get home similarly to what the ultra beasts want "just to go home" but now they will be coming back with a belly full of babies.
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My plan for the Ultras right now is basically that the MC spots an Ultra-Wormhole on the way out of Alola... maybe Necrozma can summon one?
Either way, he hops in and bangs the Ultras (if the player chooses to) but doesn't capture them since they don't belong in his home dimension and they're all fairly common where they come from
I think I could invoke some time-space fuckery to get their milk over to him, maybe he gets back to the ship and some full glasses just fall out of tiny wormholes?
An idea would be on the way to Alola while on the boat a Ultra wormhole opens and the MC has the choice to go in (while he does not have any legendries) he would arrive at a Ultra Nexus (place to choose where to go) with multiple portals to choose from and each would have a Ultra beast in a different alternate world. The worlds could be strange like no gravity places , you know places that don't make much sense but in there it is normal. When he comes to the portal that would lead home the option to leave would come up and he would know he would not be able to come back. When he does get back only a few seconds passed so none of the other legends are worried yet and all goes fine.
For the tasting it could be a bottle appears with a picture of the Ultra Beast with milky breasts and say "This is something to remind you of me and I already have plenty of children to remind me of you".