In the future? When more universities start offering courses specifically in the area, probably Human Factors and Ergonomics - essentially applied psychology and sociology in engineering contexts. Public transport, aviation, defence (both government and contracting) all employ human factors teams in vast quantities. Currently the field consists of people who have sort of just floated into the area as a result of experience and career trajectory but there is something of a push to establish a coherent educational basis for HF.
Also, this might be out of left field but a degree in International Relations (specifically a Masters from a good university) is extremely marketable at the moment, particularly when paired with strong financial literacy, due to the rapid growth of political risk and due diligence analysis in both dedicated firms (Eurasia Group, IHS, Control Risks etc) as well as in major banks and consultancies (Citi, Deloitte, KPMG, etc).