These would totally be doable with the current state of the mod: Tracts are a list of body parts. You could totally make one that starts from the vagina, except that vanilla rimworld does not define this body part, same for breasts, but if you have a mod adding privates, it is doable and simple.

This library is made to support custom digestion and release logic, but jobs are a bit more complicated to do... However, if you feel like making one, I made swallow Toil, a Eat job and a Regurgitate job. To ease whoever goes in this more complex territory.

Regression is an interesting concept. I am not sure how to best handle it, but it is totally doable. Conversion is likely doable, but I did not look into the pregnancy code yet. Shrinkage is terrifying, but doable. And absorption sounds fun and doable, it's actually something I though about, but was not sure how to balance.

Conversion into item sounds harder, I have a few ideas, but they likely would require a lot of testing...

My take in loose notes, would be as follows, on mechanics. Before I think of ability or action 'in a gene'.

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Regression can be done easy, through 'deleting/reversing' the ingested's biological age over time. Probably will want to add two end states. That being 'age regression' to a set amount/age. Or if the age reaches 'zero', the pawn just 'vanishes' as they disappear 'within' the pred, all the way.

Might alternatively lead to being stored 'within' the pawn long term? But then that'd require ideas for say, 'soul vore/entrapment', that can lead to the second part...

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Absorption I'd balance/handle as a 'soft' mix of "gain stats" from prey, if they have higher skill than the predator. And as an option, a chance/risk of 'absorbing' their traits, if the pred isn't at a 'trait' limit. Otherwise, may risk having a trait of theirs overwritten by another trait of the prey's.

Feel like absorption will be a bit hard to get right, as/under options. As stat gains could be one thing. But if it's passions? Feel that'd be a bit too oof, OP there.

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As for conversion, I'd treat it as a 'digestion' percentage if going breast entrapment ("Breast Vore"). With milk automatically being 'dropped/released' as more of the prey is liquified/digested. End result would likely be a 'serving' of milk, afterwards. But then that can be a case of 'digestive food production', as an option. (Or even 'digestive cooking?')

Likely best to consider 'testing' human meat conversion, or something of a 'Simple Meal' production for testing purposes.