One of the strong defining features which Americans are proud of is their "rugged, frank, individualistic, independence-valuing, be-your-own-man, don't care what others think" culture.
Compare that with those of the "Eastern cultures" like the Mediterranean or Southern European (think Greek or Italian countryside) (somewhat) and Asian cultures (Japanese culture being the more stereotypical "extreme" one) where they value "group-mindedness, family-oriented, clannishness, surpressing personal desires for the sake of the whole, etc." Of course, these generalizations but still a meaningful cultural comparison.
What if this was different? What if in fact, American (or how about "Western" in general) culture developped differently and took on the attributes associated with the other?
I don't think you can plausibly pursue this because your view of America (and maybe Japan too) is so mythological as to be unrealistic.
The classic of the American rugged individualist, who in fact founded the type, was Jack London; yet London was a prominent leader of the Socialist movement in the early 20thc. In many ways he was a political parvenu, he was extremely racist in his view of the Chinese, being heavily influenced by the "yellow peril" ideas of the time. Yet he made exceptions for the Japanese, who he had actually had extensive dealings with, seeing them as a nation of poets and dreamers who were being twisted into militarism.
The most distinguishing characteristic of Americans as ideologues is that we are not ideologues. Americans are practical, they do what works. It's one of the reasons that no particular dictators or tyrants have yet arisen here, dictators use ideology to justify draconian methods to force the population into the necessary ideological mold. Americans do not like molds, not even the one that says we don't like them.
It's strange that you bring up the Japanese, for they are, in this characteristic, the culture most like Americans in the world. As we know, the Japanese were never colonised by the West, despite the fact that they purposefully cut themselves off from it for over 2 centuries; because they alone of all the Asian peoples adapted their culture to it so completely. They had little folderol of inferiority feelings in that they needed this adaptation, the Westerners had a better way and they used it, no muss, no fuss. They neither turned their back on their own culture or condemned the other, they used what worked.
I don't think the Japanese are so much more collectivist than Americans, or Americans so much more individualistic than Japanese, but rather that they appear so because of their main cultural difference. Japan is possibly the most homogenous culture in the world and America probably the most diverse. Were the situations reversed, I believe the characteristics would be too.