Looking into the future, what might cause american foreign policy to become more imperialistic? more isolationist?

The rise of China could cause either more imperialism or more isolationism.

If China becomes more powerful and threatens American dominance, the U.S. might become more imperialistic in an effort to maintain its power over vast areas around the globe, for example: Southeast Asia.

On the other hand, the U.S. could respond to increasing Chinese power by reverting back to isolationism. American leaders could think that competing against China would be too costly, and that those efforts would be better spent in domestic issues.