What is "fair game" for a Master's thesis in Economics?
How your department looks on any given topic is up to them and no one here can tell you what that would be without knowing (a lot) more. In the US and many other places you need an advisor to bounce such questions off of. If you don't have one yet, find someone who is amenable to your ideas or has ideas of her/his own that you would want to explore. 

Ultimately, your advisor must sign off on your work. Both at the beginning stage when you make the proposal and at the end when you submit results. Others (a committee) may also be involved, but it is the advisor's job to be your guide and your advocate. 

Some places (and advisors) will welcome multi-disciplinary work. Others would not. Remember that this thesis is only your first major work. Hopefully it isn't your last. You don't need to try to make an international reputation at this point, but only get yourself in harness to pull for the long term, either in academia or industry. 

Advisor: good. Winging it: bad. 

A different answer may be valid in other places, of course, though I'd still recommend an advisor to work with even if it isn't required under the rules.