I think that a more aggressive, expansionist US could be convinced by Germany to join the CPs in exchange for Entente territories throughout the Americas.
In order to have that sort of political attitude prevailing in the US I think you'd have to have the US get in a brush-up (not necessarily a war or even a violent incident, just something that sows distrust) with the UK or France either during the ACW or during the Venezuelan Crisis.
This would take the cannon of American jingoism and turn it on Entente interests rather than aligning with them as per OTL.
As for post-war relations between Germany and the US, I don't think that the two would be actual enemies-- The US-Soviet rivalry OTL manifested itself in conflicts and events around the world, but could be said to have arisen from both ideological differences and competition over dominance in Europe.
After WW1, Germany and the US will have none of those conflicts. Germany is liable to liberalize following WW1, and even if it moves towards it's pre-war political disposition or towards military dictatorship, it will arise as the defender of Europe from the forces of communism, forming an important buffer between Communist Russia and the rest of Europe. In such a situation, the US would have nothing to do with Europe whatsoever, though the American public and policymakers would probably prefer having German soldiers face off the communists at the gates of Narva to American destroyers picketing Europe from Narvik to Gibraltar.
The only place where American and German interests would have any chance of clashing would be in the Pacific-- and even there, Germany probably sells or loses Tsingtao to the Japanese, while it could easily opt to sell the remainder of its Pacific territories to the United States in exchange for money which could be invested in infrastructure improvements in Germany's new Eastern European empire or to fund a few additional army corps to aid in the defense of that empire.
All told, America and Germany would likely have little to do with each other in this timeline-- Europe (and possibly Africa) would be firmly within the sphere of Germany, whilst the Americas and the Asia-Pacific would be dominated by the United States. At worst, the two countries would be essentially indifferent to each other, while in a best-case scenario he two countries might cooperate to contain and stamp out communism world wide. As two of the largest great powers in the world, Germany and the US would probably be rivals in the Olympics and in scientific endeavors such as the Space Race (the race for the atomic bomb included), but there would be little to no animosity between them and no reason for them to be rivals geo-politically.