Back in 1988, when I was reading Paul Kennedy's "The Rise and Fall of the Great Powers", a university professor came up to me and said, "You'd better read that quickly because it's all about to become irrelevant." What he meant was that the Eastern Block was about to free itself (hence a re-united Germany), the Soviet Union was about to fall apart, South Africa was about to be taken over by the ANC, and the world was supposedly shaking off a thousand years of traditional power politics in favor of something else. I remember looking at him and thinking, "I'm still going to keep the message of Paul Kennedy in mind," i.e. that big powers tend to grow until they have overstretched themselves due to ever-growing costly security responsibilities that accompany the creation and maintenance of economic power and control of trade beyond one's borders.
I think that Kennedy is about to become hugely relevant again with one caveat and that is that in his system there is usually another power (or several powers) ready to step in and take over the mantle of "great" power (such as the US did after the fall of the British Empire). The difference now is that there is no heir apparent to US power. As the US overstretches itself with attempting to control the seas, the air, the trade rules, and the energy sources of most of the world....there is no one to fill the vacuum of the inevitable ensuing decline in power and influence.
And please don't say it's China. They can't even send their warships and fighter planes outside their territory because they're too afraid that everyone aboard will defect. They have 800 million seriously impoverished people hanging around their neck like a millstone. Their entire economic boom is dependent on US over-consumption (a thing that is about to take a huge dip btw).
Russia? Their decline in population combined with huge land area spells weakness.
The EU? The EU is effectively a consensually occupied territory with not even the desire to be self-sufficient in a security sense much less be in a position to project power (even though its member countries basically invented world domination as a concept and a practice)...so where to?