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:

1. Well defined.
2. Not too big. Something someone could understand pretty well in two weeks. 
3. Academically researched.
4. Relevant to being effective.