Ironically, Japan herself could be the “sleeping giant” in this scenario. By itself, the JMSDF would already be a formidable opponent for PLAN, especially when supported by land-based air power.
Given the political will, they probably have the ability to expand their naval capabilities faster than the Chinese. China’s increasing aggressiveness at sea combined with the perception (perhaps accurate) of American weakness or indifference might just provide Japan with that political will.
One should also never forget, that if the political necessity were believed to exist, Japan could be a nuclear power in weeks. When one considers America’s declining commitment, a Japaneses, South Korean, North Korean and Chinese regional cold war is a real possibility.