I think the first thing we need to do is narrow the focus to what our national interests are (or will be) that may require the employment of our military.

The range of threats, and social collapse scenarios, that may require a military response are too numerous to list, and many are probably unforeseen such as social collapse due to disease, or global warming (global warming has already caused mass migrations in Bangladesh, and it will get worse, which has alarmed India and led to some border skirmishes).

The U.S. can only afford to put boots on the grounds in limited locations, and always maintain enough reserve to ensure the defense of other emerging threats, so not only do we need a wide range of capabilities, perhaps more importantly we need a wide range of friends with capabilities to share the work load. Rarely are there going to be situations that are restricted to one country's interests, so coalitions will be key.