Population/GDP would be a good starting point, then it'd be nice to adjust weighting automatically over time based on where the site's visitors are coming from (geolocation and previous choices).
Isn't that why we're here? I see no reason why this isn't possible (and as fast and scalable as it might be imagined) given the right serendipity of technology. Now that I think about it, weighting it for a set of countries say on a scale of 0-3 should be fairly easy and would probably fit into the :sort option provided after some tweaking.