One of the potentially most interesting, yet untapped, area of study in population biology is that of intra-individual variation. One thing we do know, however, is that this variation can contribute to cancers. When cells accumulate mutations, some of these cell lines become cancer cells due to changes to genes involved in regulating the cell cycle. It would be especially interesting to apply the theory from population biology to this somatic variation within individuals.
If any of that stuff sounds interesting, you should check out this paper from Nature Reviews Cancer. The authors summarize applications of evolutionary biology and ecology toward understanding the growth of cancers. See here for a summary. Dan has a good wrap-up on Migrations here.