The Pacific Aquadicate: An underwater civilization centered around the continually expanding Hawaiian subcontinent.
I absolutely love this idea. Reserved for Beyond Earth mod. You will be mentioned
Also this is not set in the 21st century, it is set in the 23rd (200-250 years in future)
Oh. Well, so everything is possible

but...
200-250 years ago (1764-1814)
The US and China were minor powers (US becoming a non colony, China on its way down to the Opium Wars)
US was very quickly developing since 19th century and to be honest if I was a man from 1790 I would bet on them as far future superpower. In fact, I don't remember exact person and quote but some French powerful man from this period said something like 'we helped the American people rise, and they will form an empire surpassing British naval power'
Also, in 18th century China still contributed towards ~20% of world GDP and was actually expanding. The only period of history when China was - economically - minor was 19th century and this was due to extremely specific conditions of colonial world so even then Europeans were wondering when China will awake and return to wrecking stuff as they treated this decline as periodical. So no, I wouldn't either say that 'China then was so weak, you completely wouldn't expect it to be so strong now!'
But
a) Holy Roman Empire existed
b) Prussia was European military superpower going to total domination in HRE
so it wasn't something like 'oh suddenly Germany appeared, nobody expected that'
Austria and the Ottomans were still major powers
Ottomans were in the deep decline, not sure about Austria. Also Turkey is bad example as currently Turkey although isn't superpower is pretty strong regional power with great potential
Spain was larger than Russia (controlling 1/2 Americas)
and in very deep stagnation and crisis, so de facto it's international position wasn't much better than modern Spain (especially as revolutionary tendencies in already semi - independent Spanish America were arising)
actually many people were expecting it to lose its empire in this time.
again - this is bad example for 'look, then nobody expected that!'
Japan was a hermit kingdom (like N. Korea today)
Nope. Japan for its entire history was densely populated, developed, highly cultured country. Meji Revolution and the entire imperial Japan stuff wasn't really that shocking if you look at the previous Japanese history (quick adoption of European firearms, aggressive invasion on Korea) and its entire culture and ethos. I mean, Japan went long way from isolated 'Tall Culture Civilisation' through 'Autocratic Warmonger' to 'Tall Cultural/Economical Superpower' but general rule is, these guys proved to be very capable of creating power and I wouldn't say their rise was complete surprise. Nippon is the bad example of 'look how unexpected minor countries risen to power'
Also, North Korea today is completely pathological, extremely corrupt and poor, pariah state which doesn't isolate itself because of strenght (like Edo period Japan) but because of its weakness. North Korea has 50x lesser GDP than South Korea, hell South Korean military budget is 3x bigger than North Korean entire GDP

Comparing it to Edo period of Japan, when art has flourished and cities grew, is just plain wrong
France and England were mortal enemies
Diplomacy doesn't count because in diplomacy friends and foes change every morning, not to mention the entire centuries
Now, I don't want to say that history is NOT surprising. It is, especially on personal/detailed level when you can discover really hilarious stuff

But when we are speaking about entire civilizations. economic systems and empires it becomes much more scientifically predictable. Many things can happer in the far future of BE but there are some limits.
Currently USA, Western Europe, China and Asian Tigers are so developed that it is hard to imagine they will fail the 21st century, well except from
- unpredictable random great cataclysm
- unpredictable nuclear war
- when speaking about Europe and later perspective: I expect it dying slowly for demographic reasons (but again this is predictable)
Also, just the population and great economical dynamism of Mexico/Brasil/India/Indonesia make them very possible candidates for 21st century powers.
What I am going to say... Universe is unpredictable, always something random can happen, but if we have to make the predictions for the future let's make them plausible. It is possible that Brasil will rise, or Vietnam will rise (yeah this would be suprising but this nation is currently facing extremely fast GDP growth and making reforms similar to Chinese) but vision of future dominated by Papua New Guinea would be just ridiculous
