Only a few things...OK.
Only a few things in terms of equipment. I am not knowledgeable on other aspects of modern armed forces. And of course, this is my opinion, so you are welcome to criticize it and correct me if you disagree or if I am wrong
In a short period of time (again, not specific), they "could" impose tariffs on nations intending to do business with them, or any other nation, that must transit through the SCS. This "could" be used as a source of income. In fact, under the proper circumstances, I could even see the US paying these tariffs if they thought that by refraining to do so would only hurt their own economy. These tariffs actually play right into the hands of "diplomacy", similar to the US's approach toward thwarting the nuclear effort in Iran.
I personally think this day will come in the early 2030s. China needs only a few more things- field the DF-41 en mass, field their J-20s as they wish (and possibly their J-31s), complete their carriers, complete the Type 096 SSBN fleet, and complete the H-20 and have it replace the H-6s. As they do this, they also perfect their asymmetric warfare capabilities, and their economy grows.
If war breaks out on the Korean peninsula however, it will be interesting to see if things change, in America's favor or China's.
There are more advanced things in storage which would already raise the fire-power and capability of China's armed forces considerably in a Sino-American War scenario. I'm not talking the Railgun and DEW development here, but more 'conventional' weapons, such as stealth AShMs (already known as "YJ-20") and long range hypersonic missiles like the DF-17. These things, in addition to more space-based assets of reconnaissance and communications will considerably enhance the PLA's capabilities at a relatively low price and fast pace, as compared to deploying large numbers of J-20s and H-20 stealth bombers, both of which are still waiting for their target engines (H-20 less so than the J-20).
But the best indication that China actually doesnt want to wage war against the US is the fact that the Minimum Deterrence doctrine for the nuclear forces have not been abolished to this day. China will grow their arsenal a little bit by modernizing/replacing old systems, but they are unlikely to pursue MAD, which would be a requirement for a conventional war against the US, deterring them from using nuclear weapons in response to losses suffered from Chinese conventional strikes.
Hence China wanting to choke off other Asian nation's shipping through the SCS is a totally idiotic idea. China's militarization of the SCS and the artificial islands were more of a overreaction to the feeble Vietnamese reclamation efforts that preceded them, fearing that Vietnam would begin with a Salami-slicing action to take hold of more islets and reefs in the area, which prompted China's own Salami-slicing response. Which were, of course, pretty big slices designed to cower the adversaries. Despite this, we still need to realize that Vietnam holds the majority of the reefs and islets in the SCS, including those who are naturally best suited for island bases. And the Vietnamese, nor the Chinese, have ever attempted to stop commercial shipping in the area since decades, so why the panic?
I don't see why minimum deterrence prevents China from starting a war with the US. Just the fact that the majority of their nuclear arsenal is TEL based makes it very survivable, and by MIRVing the DF-41, the amount of missiles/warheads required to deliver a full scale retaliatory strike against American military and civilian targets would not be at the same level as US or Soviet missile/warhead arsenals in the Cold War. Especially when China does not need to worry as much about a war with Russia and the US compared to... the US, which is in a position in which it theoretically could one day fight a two front war with Russia/China, the PLARF doesn't need 1000 missiles, if the Chinese wanted to bet on peace with Russia for a long time. If China is building a military that can engage anywhere however, the requirements for their nuclear forces change. Also, I didn't take into account India or Europe when I wrote this.
If the Chinese really wanted to, I think the idea of them taxing passage through the SCS isn't a crazy idea. Remember- the Chinese still do things like steal other country's intellectual property, commit various human rights abuses, and disregard their natural environment in the name of economic growth. To top it all off, they are building artificial islands in the SCS and claiming the entire body of water is theirs.
If they are even going to begin militarizing the waters, the idea of them closing of the SCS isn't that far fetched, and considering the "maximum profit" doctrine the CPC sometimes seems to use, it kind of makes sense. Although, I do believe the control of resources is the primary objective in their SCS endeavor, and that closing the seas would only be a secondary action that would come well after the Chinese military has fully modernized, or, if the US is shown to be extremely weak by some other international event.
There's quite a difference between internally directed action, like human rights abuses and using foreign IP without license, and externally directed action like choke off shipping or tax foreign vessels transiting the SCS. China's may be an authoritarian country that does not evoke much love by the West in terms of their political system, but one need to realize that authoritarian countries in an economic transition need external peace more than stable and mature democratic countries, who can be sure that due to their cyclic nature of election terms, noone actually has to take any responsibility for botched foreign policy decisions like ill-advised interventions, wars and destabilizations.
China has their allergic spots that would provoke the use of military force by the leadership, such as Taiwan and foreign suggestion that certain provinces should split off, but in terms of foreign policy, the CPC cannot take the risk of doing hugely escalatory moves that would resound back at home - the CPC has no term limits to hide behind. Whatever happens to the country, every CPC leadership has to take responsibility for. And an open war against the US because of actually less important and less vital reasons as the SCS islands is not something they would do.
That is a good point. Does the CPC think the US would risk war themselves over the islands however? If the CPC wants to be seen as powerful, and believes the US would not be willing to stop a closure/taxation, they may be tempted to begin charging for passage through the SCS.
At this moment in time however, I agree that war is unlikely, and international image, combined with internal image as you stated, is what the CPC is focusing on, and a "anti-globalization"-esque action liking closing the SCS unless countries pay would be bad.