@MH2 Also agree with the responses above... I chose a topic that I was knowledgeable about so just did some fact-checking and pulling relevant statistics. The topic was in the sector i'm interested in but not in my geographic area of interest - ultimately I felt in the short space it was something I could write well on and showed my passion/knowledge of the issue. For me, the key was to not really write a policy paper - as I did for Johns Hopkins/Princeton - but rather sort of write an 'elevator pitch' on why the policy is important and a few supporting reasons for my position.
@MPPant I also think if you can shorten that is ideal, 700 words is a lot considering the other prompts. But ultimately no one knows your application better than you - if you really think it adds something vital to your application that's not there otherwise, go for it. I used the optional essay just to give some clarifications on my transcript - explain some course names and why two were pass/fail and not graded - it was under a page. I was admitted to MIA early notification round, 169V, 159Q, 4.0AW.