How about the largest nuclear device ever exploded (the 50Mt Tsar Bomba) being detonated in London instead of the Arctic Circle?
The big limitation to the NUKEMAP is terrain awareness. Simulating terrain would take hilarious amounts of data and computation not feasible for a browser, but would produce even more interesting results. For example, even though the bombing of Nagasaki used a more advanced weapon design with a higher yield, there were fewer casualties when compared to Hiroshima due to the hilly landscape. (Well, that and bad aim due to poor visibility from previous conventional bombings nearby.)
Still, tread carefully, I've already lost several hours to this site.