If China opens a new front then the following things will happen:
Japan will get alerted immediately; China has to face a response from Japan as soon as an attack will start. China has to divide its forces.
Like Japan, Vietnam will be on alert and they can launch an offensive against China. They have a long standing rivalry with China.
If North Korea starts a new front to support China, South Korea will launch an attack in retaliation.
America has a Mutual Defense Agreement with South Korea. American Seventh Fleet will move into action.
Russia is not friends with China, There is a bitter relationship between the two. But According to a Mutual Defense Agreement, an attack on India is attack on Russia. Thus a full scale world war starts.
Also China exports to India worth around 60413.17 USD Millions. Its total trade with India amounts to 72347.42 USD Millions. India is the biggest consumer base in the world.
No country is the world is in a position to go into war with India if they are concerned with their Economic Development.
Pakistan will be excluded in long run.