Literature, philosophy, and history all win plenty of MacArthurs. Economics is unpromising, unless you study something odd. 2000 winner Matthew Rabin, for example, analyzes the economic implications of procrastination. Physics, math, and computer science are beloved of MacArthur. Chemistry is a lost cause.
Environmentalism is a sure winner.
Biologists should study evolution, dinosaurs, or primates, and little else. One in 10 MacArthurs is a writer, choreographer, artist, musician, or director in New York City. Again, pick the right specialty. Be a poet or a choreographer. Novelists, painters, and film directors seem underrepresented.