This probably doesn't scale up as nicely for density, but one aspect of my home town that I think was a strong anti-loneliness feature was the frequent presence of inviting front porch spaces and a culture of waving at people as you pass them by. It is nice to acknowledge each other in a positive way and you can strike up a convo if you're in the mood. There is a particular old man near my town who lives on a busyish road and literally sits on his porch all day and waves at each car that passes by. He himself said he started doing it because he was lonely, and members of community have said in interviews how they will go out of their way to drive past and greet him because that 1-1 genuine human interaction is sometimes the only one they get too.
That's great. It's only a shame that instead of driving by people don't walk or bicycle by, because that would allow for easier stopping and deeper interactions. Perhaps there are changes to the street design which would facilitate that, while preserving the car movement.
