Not bad ideas, I was also thinking about a future Trunks situation eventually. As for when... whenever the mood and timing is right for Mutually Assured Procreation™
there are many ways to do it tbh some games have a short period for pregnancy like 2-4weeks, other use way to time skip like magic or just straight out timeskips or you could just doe the ".... is pregnant" and them at the end you could just show the future or have a "post credid scene" with the mc and giels as a happy family or with the kids as adults and the mc and girls as senile old ppl or something like that :D