So, some time ago I found online, I don't remember exactly where, but I found someone discussing the option to add the fact that when you marry your husband (or wife) it could happen that they have already children from a previous marriage.
They were discussing this because they wanted to see incest content on this game. I'm not exactly interested in it (to be clear, I'm mostly neutral to such content. If they want to add this, then ok, but also if they don't for me it's ok. I'd just suggest to make an option to turn off it for people who could be offended or disgusted by this or even just simply don't like it or simply don't want it). But this made me think about the fact that maybe to add such a thing would be interesting, even putting aside the incest and sex part.
What I was thinking was, that by adding this, it could be an interesting thing to see such interactions between the various characters in such a situation.
I mean to add interactions in which maybe the MC can be told, when is dating someone after some time, by her partner about the fact that he's coming from a relationship in which he (or she) already had children, and so, already starting from this, there could be the interaction in which maybe she can ask to break things off, to ask for some time to think about it, or maybe tell him she has no problems with it. Maybe a MC with baby crazy or maternal (or both!) could be actually delighted to discover such a thing.
Then, also during the dating phase before marriage, I'd like to see something like her partner starting to valuate if his partner would be actually a good mother or not, especially if the children are still minors, and so starting also to valuate if it's worth continue the relationship or not. Especially also according to his (or her) traits, which would also show of course if the partner is a responsible parent or less so. Also when discussing the desires of a MC partner to have other children, if she reveals such a desire when discussing marriage plans, maybe her partner could be hesitant about it, especially but not only according to how many children he (or she) already has. Or be indifferent about it. Or finally even being supportive of such a decision if he (or she) would like to have more children, especially if he (or she) have the right traits).
Then also the interactions that would come between the MC and her partner's ex-wife (or ex-husband), both interactions on their own, so things like jealousy, both from them and from the MC, or maybe telling of how was their relationship and so reveal some secrets the MC partner have, or also being supportive of it with of course the ability to also become friends, and also maybe to add interactions in which they also evaluate her ability to raise their children. Again, all of this also according to their traits. And also of course interactions that there could between the MC and her partner's ex after the marriage between the MC and her partner.
Then finally interactions between the children and the MC. I think they could be pretty interesting to see, both with minors (I personally in this case would like to see also interactions not only with babies and toddlers, but also slightly older children or even pre-teens or teens. With all of them having different kind of interactions according to their age), and also of course if they are already adults. Things like being suspicious of the MC, or instead supportive, asking for help with homeworks maybe, if they are adults maybe asking them to go out together as friends or at least to know each other better, making small talk, telling them they are going to have a new sibling if the MC gets pregnant etc.
Of course, according to the partner age and their ex-partner age, using of course the youngest between the two, the children should have a maximug age set, so something like teen parents can have at max baby and toddler children, early twenties parents can have children at max 4-5 years old, late twenties parents can have children at max something like 10-11 years old (which I'd put for simplicity, even if slightly incorrect, in the pre-teens category), thirties parents can have children who are teens or at max who are late teens so something like 18-19 years old and so on (so the first category of parents that could have adults children), forties parents can have children who are early twenties at max and so on.
I think it would also require the ability to change such things in the custom character creator.