Working on design, virtual life elements. The gist is to make the spouse and family no different from crew, which are little different from you. Family have perceived and actual loyalty, morale, personality, moral bearing, skills, reputation, goals and life events. (No "killer AI-in-the-brain" tho)
For the spouse, you do a bunch of sidequests to get him/her. Once gotten, just as with crew, you improve the spouse's morale in order to improve their loyalty. Loyalty is the basis for bonuses like Synergy, which amps up skills between compatible characters / NPCs. The spouse should provide the highest amount of Synergy, and therefore the best boosts possible. They should also auto-manage certain property for you, defend attacks against your reputation, and generate interesting life events which give you a chance to risk resources for some gain.
I think the virtual children might be more interesting in that there's a sort of grooming aspect that's possible: Basically, character creation by responding to life events. What the child excels at, how they view the world and factions therein, and what goals they have will heavily reflected your input.
I should stress that the way Synergy and life events works, you probably won't want to leave the spouse at home all the time. Most Synergy bonuses require the two characters to be on the same level; so your spouse could be your partner in crime, trained like you to fight, stealth or trade their way through the universe. It should be an interesting tradeoff, though, because if they stay home they can improve property, defend your rep and get you contacts / missions.
The life events should take care of those who want meaningful story. The leveling should take care of those who want gameplay.