I remember in one of my first saves, I had one of my characters marry her. I think the BFFs household is quite iconic in that regard. Almost every new player gets themselves married to Travis, Liberty, or Summer.
In my current series, she and Summer are supporting characters in the base game arc. After they temporarily leave their house to overcome Liberty's fear of fire off-screen (from setting the grill on fire one too many times), they pretty much vanish while Travis lives with my two OCs as a main character. As far as I remember, I think Liberty and Summer adopted a baby together thanks to one of my Sim's Neighborhood Confidante aspiration. I like to imagine they're living happily together as a couple.