nice! you could also make versions where the border has to be on the coast or not on the coast. Eg Brazil-Venezuela border isn't on the coast, Mexico-United States is
Interesting idea! I think each pair's total length of coastline would be a more intuitive thing to list here. Average rank is such an abstract concept.
Not really, since they're ranked by average rank. If average coastline was used, then larger countries would dominate (anything with Russia). This way, both countries have to be fairly coastal.
Spotlight worthy!