Not to be discouraging, but doing something like this would be a massive undertaking if you wanted to do it right. Keep in mind, this game is, in a lot of ways, a social simulator. It’s already got a whole load of social interactions happening. Add this would put in a whole new layer of considerations when NPCs are interacting, it would drive the complexity through the roof.
That’s not to say it shouldn’t be done (I’ve been mentally toying with the idea myself), it’s just not going to be easy.
I have thought, on a couple occasions, that what might be better is to break that problem space into some distinct units and see if those issues can be sorted out on their own. With that in mind, here are a few possible mods that might lead in that direction:
Best Friend: Create a mod where a hearthling can, at very least, have a best friend. They would be more inclined to help their bestie (with work, with fighting, with finding food, etc.) And the mood of their bestie would have a more significant effect on their mood. (Besties can eventually be spouses).
Reproduction: Without bothering with all the niceties of actually having a relationship two hearthlings (any two hearthlings in my head, spoiler: controversy), would be able to get together and perform some sort of ceremony (maybe a dance?) and up would pop a new hearthling out of thing air. The new hearthling would have states/characteristics randomly selected from the parents, and the new hearthling would be about half the normal size (not a baby, just small). They would grow up over a cycle of seasons.
Note, this one is going to be difficult to balance because the entire game hinges on the idea that as you advance you get new hearthlings. So that would have to be considered if this mod is going to be fun for players.
Ceremonies: It would be cool to have a mod where any sort of ceremony can be performed. I think it could piggyback off of the concept of a “celebration” in town. The biggest issue with this would simply be setting up events that trigger ceremonies (like two hearthlings getting married). Honestly this one would be a fun one to have just all on it’s own
Ownership/Co Ownership: There would need to be a mod set up to create the idea of ownership beyond just assigning a bed. IE., we would need to give hearthlings the concept of being able to “own” an object and to care if anyone else is using it without permission. Then would would need to introduce the idea of co-ownership… (and then maybe the idea of lawyers and nasty breakups for when things head south, right?). Lol, this is probably the least necessary of the ones I’ve mentioned, but if my divorce taught me anything, it’s that this is a central concept in a marriage
These are just a few that I’ve had dancing around my head as I’ve been going over the problem space mentally. There are likely many many more. But it may be possible to create the idea of marriage if enough people were interested and had the time to devote to the concept.