Superpowers aren't going to use nukes on each other, due to the threat of MAD. Terrorists may be able to steal a nuke and deploy one for their nefarious reasons. Lets say a terrorist group gets a briefcase nuke from somebody, then blows it up in a major city.
500+kt is strategic level, if superpowers used it we're all screwed. But the typical nuke is tactical-level, or ~10kt or so. And tactical-level nukes could be theoretically stolen by groups (there are way more tactical nukes than strategic ones).
We're looking at ~half a mile radius that's blown up, but otherwise a survivable blast. There are rumors that as the USSR collapsed, it lost track of a number of their small tactical-nukes (even if they kept good tabs on their strategic nukes).
It seems far more likely that the next nuke to be used will be a smaller tactical nuke, probably in the hands of terrorists.