At the beginning of the game Victor chooses a woman to have a successor (our Steele) with, got me thinking about who I'd pick. I'd pick Fisi or Bianca but neither of them have pregnancy content, and it's unclear if Jill will so now I'm wondering who I'd pick from the available options. Probably Roxy? Stella or Nastizia too but in their cases, Stella's got her own major business and Nasti is royalty. Depending on how you choose to view it, those could be benefits or conflicts of interest.
Fisi was originally planned to have pregnancy content(Heat Sex is a thing which implies she's fertile), and Bianca iirc is literally married. So only Fisi would be a valid option.
Fisi would be my immediate pick as well. She's just a nice, normal girl, which can be much more said than literally the rest of the galaxy. Sera is also a good pick for both sides if Steele dominates her(sub Steele I feel like would have a bit of a one-sided relationship which could be bad for the kid), as she shows ambition and does in fact have a softer side to her when it comes to her own kids, so hearing that one of her kids would be the heir would definitely straighten any kinks in her out. Kase is also a good choice; a subby kitty-boi would be a great father. Gel Zon would also be a good father, experienced fighter and a chivalrous personality. There's also just so many other options like Sam, Evening, the rat girls, Briha, and Reaha, but they're not just simple people. Briha would def end up as a pick though because she'd get her problems solved and also would guarantee the survival of the myr race if I decide to blow up the planet later.
Of course, all of these people are nobody in comparison to the Frostwyrm on Uveto. Repopulate the dragon race with them, and you get an empire that ends up greater than Steele Tech by leagues with high INT(which is guaranteed with Mhen'ga Syri intelligence grind), to the point that your resources keep it afloat after your return to society. Needless to say, your heir would be way better by default, and might even start with psionics.