Finding a good research proposal

Which subject area is this?

international relations

You cannot help what you are interested in, but there is no reason why what you are interested in should not be critical or practical, so to speak. In terms of international relations, certain issues are more likely to be of short or medium-term importance such as (1) global warming, (2) dwindling natural resources, and (3) nuclear disarmament. The political system of Belarus is an important issue for its neighbours, and east-west politics generally, but it is not something most will care about in the near future unless attention is drawn to it; the 2008 South Ossetia war is another Slavic example in the same vein.

But for the three examples above you might consider:

(1) – Can it be effectively tackled? Can a universal system be deployed? How would a partial system be implemented in the context of developing countries such as China and India?
(2) – Are resources dwindling, or is it about market control? Are we resource dependent? Are particular countries overemphasising their political importance, because of resource control (i.e. the middle east)? Are there any positives to dwindling natural resources, or increased market control (i.e. the economic development of the middle east and Russia [notice how both were just awarded the World Cup, for example])?
(3) – Is this an international issue? How can this be an international issue when only a handful of countries possess nuclear weapons? How can international relations function effectively with such disparate differences in defence budgets? What would happen if everyone disarmed?

This is not my area of expertise so I apologise if I have just butchered it; my understanding of the subject is that it is a interdisciplinary area with a focus on politics.

Well for someone who is not studying the subject, you understand it pretty well

I was thinking that doing a research project on emerging economies, Yemen... would be more interesting to admission tutors than researching about say, global warming which you mentioned.

but then the problem is that some countries/topics are important now, but maybe not in a few years'time

You should choose something that you are interested in and go with that as you will have to spend a long time studying it. No point doing something that you're not interested in (although the proposal that you apply with may not be the same as what you actually end up writing). You should be looking for a university that has a supervisor who may also be interested in that area. You will need to find the academic side of the issue too, and frame it in an academic way.

Most tutors aren't looking for projects on fashionable topics such as global warming, but are interested in their subject area. A tutor who specialises in the Middle East will remain focused on the Middle East.

I think that Yemen is probably a very good, and quite under-researched area. It's becoming increasingly important in a global sense. There are a whole range of topics you could cover I would think.