Tyler Cowen offers six, not mutually exclusive. For example,
4. Energy becomes very cheap, destruction is easy, deterrence is difficult, power decentralizes, and we retreat to medieval-style fortresses.
In my view, the two most interesting variables in forecasting the future are (a) the path of biotechnology; and (b) the ability of the center to hold in U.S. politics.
For biotechnology, the question is whether the new pills (or what have you) do nothing at all, primarily help the disadvantaged, or mostly add to the advantages of the elite. For politics, we could have the center hold, we could succumb to a strong demagogue, or the central government could suffer a sudden loss of authority (what if investors lost confidence, so that the Treasury could no longer borrow?), with people (especially those with talent) either leaving the U.S. or setting up alternative government systems on its territory.
The most likely scenario is that the center holds and biotech does not do much. Also, I think that if biotech primarily helps the disadvantaged, then the center will hold.
The other possible cases are:
1. Biotech does nothing, and a strong demagogue takes over.
2. Biotech does nothing, and the U.S. central government loses a lot of its authority.
3. Biotech helps the elite, and a strong demagogue takes over.
4. Biotech helps the elite, and the center holds.
5. Biotech helps the elite, and the U.S. central government loses a lot of its authority.
I think that (5) is the second most likely scenario. I think that (1) is the third most likely scenario.