I don’t know. It sounds to me like a difficult storyline to have as a continuing series.
If some cataclysmic event caused this to happen, then that new president would appoint a Vice-President under the 25th amendment, then appoint a completely new cabinet, as would any incoming newly elected president. And with some exceptions, the president’s cabinet would be confirmed by the Senate, and life in the government goes on. Then at that point, isn’t it just back to business as usual????
I suppose it’ll depend of the backstory going into this, wouldn’t it?
What if the previous President was into some kind of high-level chicanery?
What if the new President were EXTREMELY unpopular among the citizens? Or, how about unpopular with foreign governments?
You see....it all depends on the backstory.
They could even have one of the higher-ranking cabinet members be severely injured rather than killed. Maybe in a coma. Then that person could come back from the edge, and if he's a political opponent of Keifer's, then that could cause additional tensions and conflicts.
Anything that kills so many heads of state is an act of war.