Loool ehehehehe, so Japan wants to challenge the west/U.S? Good luck with that.. Just because we(west/U.S) favour Japan now as ally doesn't means Japan should take us for granted. If Japan tries to do what you just said: I.e go it alone irregardless of U.S/western cooperation , then I can assure you that you will be the next China or worse. I'm sure Japan doesn't wanna be viewed negatively by most of its smaller poor countries in Asia like Vietnam, Philippines, or small but richer ones like Taiwan, etc do we?
The west/U.S can easily make Japan become a big bad expansionist/aggressive power.
So be careful of what you wish for nihonji, since our governments in the west/U.S are still by far the rule makers in the world basically.
As for China, our friend Yizhi is right, the U.K and France have far more power projections capabilities/experience/advanced than china. China doesn't expand for now not because they don't want to per se, simply because they are still an immature power, reason they need 'more time' at least 15 to over 20 years as yizhi and other Chinese members here said to mature.
So projecting power for them like the U.K,France have been doing for decades is a non starter, I'm not even mentioning the U.S who we all know is in a different league altogether. China needs to first match our (U.K &France) power projection capabilities before she can even dream of challenging the U.S. still a longgggggg way to go for that to happen. Like yizhi said maybe around 2030 upwards if we want to be optimistic.
That remains to be seen though, since many things can happen that can slow China's growth down as well, we never know. So let's just wait and see.
Finally Japan should tread carefully while working/cooperating with the west in any foreign military adventure it might want to take on. Believe me that is the best solution/choice for Japan , doing otherwise will mean trouble for Japan. So Japan needs to stay the course of its alliance/cooperation with the west like it has been doing since the end of the second world war.