China could gunboat diplomacy through Asia, holding Japan and Korea hostage with firepower.
There's no reason at all to believe China assert itself like this. They won't even kick out the world's #1 child rapists (white males) from "teaching" ESL and they still hand out scholarships to unqualified candidates throughout the developing world. I take no pleasure in saying this. An assertive China is a laughable fantasy. You should instead hope and pray they don't go full eunuch and import the world's trash for "win win cooperation in the new era of great harmony".
One more thing. Why do you describe China's possible future actions as "gunboat diplomacy" on "hostages"? This is what I mean by East Asians are just insanely bad at soft power. There's no such thing as Chinese "gunboat diplomacy." So far, there is only Chinese Santa Clausing where it gives away hundreds of billions in aid, technology, education, etc. And the recipients aren't even good children. They're often retarded racist pricks. China is doing the thankless job of lifting the developing world into modernity. Yet they somehow manage to make themselves sound like a demon while being a angel. This is the exact opposite of whites who sound like angels while being demons.
East Asians
call foreign trash "expats"
describe imaginary future Chinese actions as "gunboat diplomacy"
call Western subversion / meddling "intervention"
call overthrowing democracies "regime change" or "color revolution"
call their China's allies "attack dogs"
call Western racist genocidal imperial fascist oligarchies "democracies" or a harmless inaccurate term like "hegemony" instead of fascist empire
call Western invasions "misadventures" or "interventions"
call Russia's defensive war an "invasion"
call China an empire / imperium despite not acting like an empire at all.
call Western attacks as "confront China" which implies China is a bully that needs to be "confronted" by the "victims"
call Western economic war as "restrict" China - even the NYT calls out usa's economic war
Some of your (and it's not just you. there are many on this forum) language usage is a product of western brainwashing. When I point this out (with many examples) people will look at me like I'm crazy.