I don't think there any other ideas for an American UA that is as perfect as increased growth from trade routes. It fits American history and uses BNW mechanics in an interesting way. What else needs to be said?
The problem of course is the AI will invariably have more happiness until the ideologies kick in and the playing field evens out abit. Even then you have to follow the dominant ideology or become the dominant ideology yourself to reap the happiness rewards and inflict 'dissent' on others.
For that to work, there has to be a tourism/ideology component to it. Even then it's only awesome on the final half, just like the B-17 is awesome but games by then are already won, especially on the higher difficulties.
The question then is
Do we want more 'highly situational' UA to replace another highly situtional UA the Americans have?