I'm not really interested in marriage or kids or any of the like in the game.
I still think though it would be a good addition if we could somehow build a relationship with an npc, maybe make them rather fond of you if you were fond of them. They'd great you differently, and treat you differently, whether you be friends with him or her, or maybe a bit more. Perhaps even live in a house you own.
I see no reason to go into marriage and kids and stuff though, that's a whole nother can of worms.
I just grew tired in Oblivion, where every character greeted you "I'm honored to speak to the Champion of cyrodiil, how can I help?" Or any number of other greetings that were very formal and slightly awkward, even if you had met this person several times, and even if they were a guild-mate or neighbor. I just found it awkward.
In fact I don't think I felt like anyone was really my friend in Oblivion apart from a certain few, such as the maid in the Skingrad house. She greeted you cheerfully, and like she actually knew you. Not even Baurus or Jaufree after the main quest greeted you like that. In fact after the main quest finished you might had well been another stranger walking up to those two, they greeted you with no emotion.
So I feel like character interaction in general needs to be upgraded in some way. When a characters disposition raises they should act more naturally around you, like they know you.