I can't speak to the Incan part, but the American UHV can certainly be made more difficult, particularly if the 1700 AD tech problems are resolved. Currently, a reasonably aggressive player on the 1700 AD start can clear the Monarch/Normal UHV easily, while building wonders to spare. In my last run, to try out the new wonders + lategame features, I was able to build basically every historically American wonder - the four that you are required to build are not that difficult all-in-all (it helps a lot that most are on the bottom half of the tech tree, which the AI does not prioritize). For the 2nd UHV, perhaps changing it to require the four currently named (Empire State Building, Pentagon, United Nations, Statue of Liberty)
plus any other six Industrial/Later wonders.
For the 3rd UHV, perhaps something like - "
Pax Americana: Between you and your vassals and defensive pact partners, have at least 75% of global GDP and 50% of global military strength in 1990." Basically reflects the absolute dominance the First World (in the Cold War sense of the phrase) had at that time period. The numbers are pretty arbitrary, but looking at
https://en.wikipedia.org/wiki/List_...orical_GDP#By_average_values_of_GDP_(nominal) they don't seem unreasonable historically at least for GDP. They would obviously need to be adjusted for gameplay. Also leaves open several options to getting there - building tall in America proper, or taking an early Manifest Destiny approach across the Western Hemisphere to either puppet or control directly South America for further land. Or, one could turn the Cold War hot and just demolish every possible competitor. Lastly, it also does not specify a particular civics setup, which addresses the hope addressed occasionally to not railroad the USA as heavily into certain civics.
I'm sure there's more creative options out there for the 3rd UHV, akin to the 3rd Russian UHV, but I think this is a reasonably simple approach.