The USA is at 'peak' , we can roughly say this, not because of anything America is doing, but because the 'rest of the world' is coming online.
50 years ago there was 'America', the 'Soviets' and 'Europe'. Everything else was a footnote.
What we've seen with China 'coming out of nowhere' in the last 25 years is about to happen 4x with the rest of the world.
America and Europe's share of global economy is falling fast, and it simply has to do with electricity, plumbing, democracy, knowledge and productivity gains in the rest of the world.
India is coming on a little slowly, but surely.
Pakistan is 180 Million people. Nigeria almost that - and they have oil.
Africa is a billion people.
There are a lot of Asians outside of China, India and Japan.
As they start to become connected to the global economy, relative power shift will happen.
I don't see the emergence of any new 'superpower' and America will likely remain the only 'superpower' in the classical sense, but it won't be as powerful as it is today - relatively speaking.
If you look at the growth of Japan after WW2 - a period of fast growth, then slowing down as it approaches parity with the west - this is what the underdeveloped world will look like, but a little slower.
Also - a very important point - a lot of real growth is not measured in the GDP.
For example, the value that is created by governments is measured simply in 'government spending'. But $1 of gov. spending hopefully creates more value than $1!
For example, if the US Gov spent $50B and created this amazing national public transport system that everyone used, and cost next to nothing - the 'economic surplus' would be massive, but the GDP would directly be down. Why? Because the only thing that would count towards the GDP would be the $50B spent by the gov, not the $Trillion in value derived from it. Obviously, there would be indirect business benefits ... point being - the GDP is not necessarily the best way to measure things for many obvious reasons.