The following could be a preliminary list. Are there others that should be included? And how should these threats be ranked?
"Soft" Coup - E.g. a remake of the election's overthrowing attempt that culminated in the election de-certification push followed by the assault on the Capitol on January 6, 2021. Or massive gerrymandering. Or radical changes to the manner of appointing Electors... The possibilities are endless.
Military-Civil Coup - It has never happened in the US before (but neither was a "soft" coup attempt deemed likely); could it happen in the near future?
Civil War 2.0 - Whether it's the secession of either "Red" or "Blue" States, as some high party officers and talking heads seem to be encouraging, or explicit calls to an all-out war, how likely is it?
Great Depression 2.0 - Fortunes have been made predicting the next big economic downturn, but could it happen? How to prevent it?
Heightened social unrest - Can the faultlines in the American society deepen and in so doing erode the rule of law and cause widespread and escalating violence?
Disasters - Climate-change-induced, health crises (pandemics), domestic terrorism, international terrorism, war... Pandora couldn't keep them all boxed in - What can be made to prevent them? And how to prepare for them?