One thing we could do pretty easily is use IP geolocation to highlight "meetup in $place" posts for readers who are accessing HN from $place. I'm not sure if this would be extremely welcome or extremely off-putting.
I mean, I'm sure it would be both, for different users, but I'm not sure what the percentages would be.
It'll also be good if locations/cities couldn't be monopolised by a single large meetup. I attended a few HNLondon meetups back in 2012, but came away with nothing.
Smaller meetups might work better for actually getting to know people.
IP geolocation gives latitude/longitude. Maybe we could sort such posts by proximity. Whether they'd be comments or top-level posts would depend on the format I suppose.
I mean, I'm sure it would be both, for different users, but I'm not sure what the percentages would be.