I've seen this bandied about recently, and there's at least one erroneous account of Congress attempting to pass into law German as a national language. (In reality it was a bill to print laws in German as well as English, probably to aid the 7% of Americans that spoke German at the time) But is it plausible? And what circumstances would have had to arise for the US to legislate German as the national language? A particularly bad bruising from Britain? That aforementioned law passing? A close alliance with Prussia/Austria?
I'm no German nationalist, but I do think a German speaking USA would be particularly interesting, it would have some interesting side effects. For instance, the Canada-US dichotomy would be much starker, and in my mind a German speaking USA could set the groundwork for a more 'European' America, but I'd like to know what you think.