The most realistic scariest thing I believe in is the United States will be hit with a nuclear bomb. I hope this doesn't happen obviously, but unfortunately I do think we are going to get hit by one in our lifetime. I don't know which country or terrorist group would do it, but I feel it is the inevitable. There is no way these countries are spending billions of dollars and putting this much time and effort just for fun.
This is a groundless fear imo.
These countries, with dictators that have nuclear programs, do so, for their own "freedom".
If you have nukes, it means the USA wont invade you, and overturn your rule, in the name of "democracy".
The terroritst groups you mentioned are the real threat.
Its not the small countries with nuclear programs that are a worry, its the single madman that only wants or needs a single one.
Still how would you even get one on US soil? I doubt that'll ever happend.