To be plausible, I've read, a novel must avoid reality.
What novel, for example, would start the 21st century with al Qaeda's attack on America? What novel would have an American President spend a trillion dollars and hundreds of thousands of lives attempting to recreate Grenada in Iraq while tossing aside the Laws of War?
Reality is not as cautious as writers. And so the 21st century began with the end of American exceptionalism. More than a decade later, we've got the feel of it. Not of the whole century for the whole world, but at least of the years from 2010 to 2040 for America.
What are the main themes? I'm sure I've missed a few, and of course there will be surprises, but here's my starter list:
- Demographics 1: From 2010 through 2040 America will be divided between an increasingly senile, largely white protestant, cohort born before 1964 and a relatively diverse and secular cohort born after 1964. The many "fiscal cliff" fights to come will reflect this shift.
- Demographics 2: Even Hispanic birth rates are falling. The relative cost of children will continue to increase even as 93% of income growth goes to the top 1%. Given Demographics 1, American will have to attract millions of new immigrants -- even as the American brand struggles to recover from the Bush regime.
- We are in the post-AI era of both great wealth and mass disability.
- China and India - whether they thrive or struggle or both it's their story now.
- Nuclear proliferation: More nuclear weapons, more launch systems, more hackable targets. Iran, North Korea, Pakistan, India ... and so on. 
- The cost of havoc will continue to fall. I was really torqued about this in the months after 9/2011; I didn't see how we'd avoid turning into a surveillance society (at best) or an authoritarian state. Well, on the one hand we did turn into a surveillance society, but on the other hand we haven't seen any home-brewed bioweapons yet (except for this one of course). I still think this problem is not going away, and neither is the surveillance state.
- Innovation gap: AI aside, there's something wrong with the engines of our ingenuity. Maybe we've done all the easy stuff, maybe it's the NIH and the scientific-industrial complex, maybe it's because so much talent is wasted playing finance games, maybe it's the triumph of the Corporation and its IP laws, maybe it's all of the above and more. This gap is a bigger threat to our future than social security or even health care expenses.
- Winner take all: It is insane that growth in our economic output is going to such a thin slice of our population -- 37% going to 15,000 households.
- The triumph of the mega-corporation: For better and for worse, but mostly for worse, the large centrally-planned Corporation will rule the American economic landscape for decades to come. Elephants have made the ecosystem of the African Plains, and Corporations have made the laws and accounting systems of America. Citizen's United will shape the decades to come.
- Weather adaptation: The big devastation from CO2 emissions is probably in Book Two, but Book One will have big enough problems. We will eventually adopt carbon taxes; driven both by need to raise revenues (see above) and by the slow acceptance that we've whacked the Earth pretty hard.
- Good enough health care: After exhausting every other option, the US will come to accept good enough health care.
- No more big US wars: Being old and worried about budgets is not all bad.
 Oh, yeah, and what novel would have a bioweapon attack follow 9/11, be used to justify a major war, and then be completely forgotten?