Next war? Probably none for a long time. Secretary Gates said a future president would be crazy to get involved in another land war in Asia. Obama then precociously started an air one in Africa, but the wider point that direct American involvement in wars is going to be avoided in the future is probably still true. This doesn't mean we won't keep launching strikes in Pakistan from much reduced bases in Afghanistan for many years to come, but it does mean Iran you won't see US tanks rolling through Tehran. Everything's gonna be on the DL and be much more of a balance of power game through a still stronger navy anfd lots of support for key US allies (the arms deal with Saudi Arabia is a clear example).
But in the spirit of the thread, I'll guess a deeper involement in the Mexican Drug War if things continue to deteriorate. Who the next President of Mexico is and what policies he pursues will determine what happens there, and if the Mexicans can't reassert government control i nthe worst areas I fear America will be drawn in somehow. That would be a dreadful scenario, but unfortuately I think it's quite possible.
"Eliminate the possibility of the U.S. going to war with any nation that could actually fight back effectively. So definitely NOT NK or China"
Oh mapleleaf. Leaving aside China (which, while it would be a serious war, the United States could handily win conventionally), North Korea poses no conventional threat to the America. The main concern against fighting the North is what happens after the government is destroyed and millions of refugees flood into a ruined Seoul and a freaking out China. The nuclear missiles, while a serious, serious threat, would likely be aimed mostly at Japan. We also have the technology to shoot them down pretty well. It would still be dreadful, but to say North Korea could fight back effectively is quite wrong. If there's a Second Korean War there will be no North Korea at the end of it.