I would say that it is in progress, or that at least, the American empire is in serious decline. Our over-reliance on an unstable resource (oil), our overconsumption of other resources, our population growth, and most seriously our debt and deficit spending (not at all a new issue, but a 60 year habit) may well be our undoing.
China is the de facto choice for a successor empire, but I disagree. Their economy is intimately tied to ours, and our continued consumption. They cannot maintain their economic growth if America's is in decline. Their population is massive and they, also cannot get the oil they need to continue expanding.
I think for a time there will be a power vacuum in the world after the American empire falls.
I'm with #2 on this one. That doesn't mean other countries won't grow in stature and power in whatever ways you might want to define that, but America has many things inherent in its history and structure which make it difficult to supersede. We're a country of law, we're a country with a strong work ethic (though it has certainly weakened over time), we're willing to fight for those who can't fight for themselves, we're not afraid to fight when right is on the line, and we are united (despite our temporary glitches) in our loyalty to this country. Lots more, I'm sure, but these are at the top of my list. More than anything, though, I believe America is on top, so to speak, because of these things, not because we're so very driven to be number one. Easy to say from the top of the mountain, I know, but true nonetheless, I think. Time will tell if things will change, I guess.
This is, of course, a complete matter of speculation. We cannot know this for a fact, no matter what. There is also the problem of definition. How, in your mind, do we know when one country is "Number 1?"
China is the country that people talk about the most. However, it still has a GDP that is lower than US GDP even though its population is at least 4 times bigger than that of the US. It is not likely to overtake the US in GDP (let alone in GDP per capita) anytime soon.
Militarily, a country that is that much less rich than the US is not likely to be able to afford enough military hardware to overtake the US any time soon either. China, at this point, does not have the military power to project across the globe the way the US does.
If I had to guess, I think that the US will remain "number 1" (however that is defined) for the rest of my life (I am 40 right now).
Different countries and people have wielded, or at least appear to wield great power at different times. This includes cases like that of Alexandre, Genghis Khan, Great Britain, Napoleon Bonaparte, USA, Hitler and Russia. There were really no indications of their power, before they actually achieved it. So, I do not think that it is possible to predict which country is going to become the next super power, or when such a country will emerge. As a matter of fact it is not possible to predict even the source of such power. Right now when we speak of super power we think mainly in terms of military power. Economic power is now emerging as a clear option to the military power. May be in future, information power, or spiritual power, or some other presently insignificant type of power may become the decisive power.