My guess is a case that involves the right to carry a firearm outside the home. My read of Heller and McDonald says the right exists, but it's far from settled law.
Kavanaugh has written that handguns are protected, and most handguns are semiauto. He then said that from a legal standpoint, there is no difference between semiauto handguns and semiauto rifles. So according to him, semiauto rifles are protected. That would make some interesting fodder if someone challenges an AR ban.
Magazine capacity limits are also another interesting possibility. A magazine is a functionally necessary part of a semiauto firearm. It is therefore as protected as the firearm it fits into. If the government cannot specifically show that such equipment poses some unusual danger, or is not it common use, those laws are easily vulnerable.
My theory on why we have not seen new firearms cases is that nobody on the Court wanted to grant certiorari since they weren't sure they could get a majority behind a good decision. Somewhere, tonight, Alan Gura is quietly plotting his next six chess moves.
