narrowing down the topic

The subject of my term paper is idioms. The field is so enormous and I'm just lost. At last, I came up with an idea of researching a particular class of idioms and seeing if they are driven by metaphor, metonymy, hyperbole or euphemism. Now, I need to decide what class of idioms I'm going to examine. What could be of relevance? Though the topic itself seems to be interesting, I'm still in doubt about the purpose of such a research... I'd appreciate any ideas. Thank you sooo much!