It is hard to say what impact, if any, Islam will have on American religious pluralism over the long haul. I can envision three different impacts. I will outline each scenario, and leave it to you to decide which you think is most plausible.
First, it is possible that Islam will make American religious pluralism more diverse and more tolerant. To this point in our history, our religious pluralism has (at its best) typically extended only to Christian sects that are not too far out of the mainstream and to Jews. Even this took a while as Roman Catholics and Jews have suffered from prejudice at times in our past. If America’s Muslim population grows and becomes more visible, it would be the first time that a religion that is not clearly Judeo-Christian (though it is, of course, derived from this tradition) would have gained acceptance in this country. This would give us a truer kind of religious pluralism than one that accepts only a narrow range of sects.
Second, it is possible that Islam will actually reduce the amount of religious pluralism in the US. It would not do so directly. Instead, the reduction would come as a reaction to the growth of Islam. In this scenario, Islam would grow and become more visible. The majority of Americans would see it as a foreign and hostile faith. They would resist its spread, attempting to do things like making it harder to build mosques in American cities. They would make Muslims feel un-American. In this climate, it would be harder for non-mainstream religions to be accepted. This would reduce our level of religious pluralism.
Finally, we can argue that Islam will not change American religious pluralism to any appreciable degree. In this view, Islam will just be one more religion that exists in America without being noticed much by most people. Muslims will be a very small minority that goes unnoticed and, therefore, does not really contribute to American religious pluralism much at all.