Here are the last time any of these things happened:
1. Last American man to win the US Open: Andy Roddick in 2003. (Second-last: Pete Sampras in 2002).
2. Last Australian man to win the Australian Open: Mark Edmondson in 1976. (Second-last: John Newcombe in 1975).
3. Last French man to win Roland Garros: Yannick Noah in 1983. (Second-last: Marcel Bernard in 1946).
4. Last English man to win Wimbledon: Fred Perry in 1936. (Second-last: Fred Perry in 1935).
5. Last teenager to win a Grand Slam: Rafael Nadal at Roland Garros 2005. (Second-last: Pete Sampras at US Open 1990).
I include the second-last time to make two points:
a. Nadal is the only teen Slam champion in 28 years.
b. Both France and England have had long, long droughts at their home Slams. Australia's is getting up there, too. So, I voted for option 1, as there will likely be another generation of dominant American men at some point, whereas there might not be for any of the other countries. At least not in a while. Kyrgios has more chance than any current American man, but there's a pretty good chance that Kyrgios will not win the Australian Open in his career, so I didn't regard him as the decisive factor. And Alex de Minaur is not winning a Slam.