Pure idioms

I'm in the process of labouring (such a pain) a research question for my term paper. The major field is phraseology (term used in russian lexicology) or idiomology (the term used by western scholars). The problem is that i cannot narrow down the topic . I've been thinking of doing a research on pure non-literal idioms, but what could be the purpose of such an investigation? Which methods to employ? Please, if you have any ideas...