So, I think the problem is that meet-up threads have sharply discontinuous value; hearing about Meetups in NY does me no benefit at all, whereas hearing about one in Oxford or London would be invaluable. Any given top level will be irrelivant for over 99% of the daily readers, but the utility from the few who turn out might well make high-prominence still the best option.

Some sort of automated system based on location could be better, but anything requiring coding should probably be rejected in favour of social solutions, ceteris paribus. Also, there'd be issues with people moving area.