One big thing would be the US ever becoming interested in anything outside its Sphere of Influence after Theodore Roosevelt. Wilson was kinda internationalist but he wasn't in power long before dieing of that stroke. I mean the Democrats are always "Focus on the US first above all" while the Reps are torn between the isolationist Taft wing and the interventionist Roosevelt wing that always comes to power under a Democratic or Taft wing Congress.
As an aside its amazing how much the Presidency looks like an elected monarchy anymore. I mean with 4 Republican Roosevelts (Theodore, Quentin, Franklin Jr.*, and most recently Kermit*, 3 Democratic Kennedy's (Joseph, Robert and now JFK Jr.), we've been under the rule of two families for more than 50 years!
If the US tried I think it could become a diplomatic superpower and not merely an economic superpower and considering that the US's GDP is about twice that of Japan's* its diplomatic influence would be similarly outsized. Given the fact that the US has been theoretically against European style Colonialism post Spanish-American war if it actually pit itself against Colonial powers things would be happening.
OOC: Franklin Jr. and Kermit are hypothetical kids of course. The Dems and Reps are much more of a mishmash of political viewpoints with the Dems being more fiscally liberal/progressive but retaining the South and its social conservatism while the Reps are socially liberal and more split between Taft's laissez-faire fiscal policies and the Roosevelt progressivism.
World GDP is a decent amount higher without two world wars and a much less crippling Great Depression (1930's were difficult but not disastrous). Here's what I would think would be a good shakedown
World - 85-90 Trillion (vs 62 Trillion)
USA - 18 Trillion
Japan - 9.5 Trillion (of course including Korea and Taiwan boosts its numbers quite a bit)
Russia - 8 Trillion (Includes a lot of area that would be Manchuria / Northern China.)
Germany / Britain - Around 6 Trillion Each
China - 5 Trillion (much smaller population than OTL without Mao, around 700 million people)
France / Brazil / Austria Hungary - 4 Trillion
Italy / Spain / Canada / Mexico / S. Africa / Australia / Netherlands / Ottoman Empire - 1-3 Trillion