Yes it could be expanded. Because the Foursquare api doesn't provide historical data it takes a few weeks of monitoring the venue before I have useful data for it however
One immediately apparent reason would be API rate limits from Foursquare. It's easy/practical to query for "who's here" for a limited number of venues a few times an hour, start doing it for hundreds or thousands of venues and you'll start running into problems...
Yeah, make it a smart phone app, charge $2.99 for it and include everywhere that foursquare supports, money to be made! Although maybe foursquare could just release a similar feature.
