Two that come immediately to mind:
1) Cognitive enhancement, covering both the various nootropics and more "traditional" methods like exercise and getting enough sleep.
2) The paleo diet
(If these are too large, covering one particular facet would still be valuable. Ex: one particular class of nootropics, or the lectin-allergy hypothesis)
I think I want to stay away from nutrition; there's such a bad track record of controversy there.
The effect of sleep and exercise on cognitive function are good topics.
The last prize, for the best literature review of spaced repetition, was moderately successful, inspiring a pretty good review of the academic literature on spaced repetition. I am interested in experimenting more with prizes, but I would like to get input other people's input: what are other good topics for future prizes?
The topic should be: