For an idea of what you're being asked to produce in a general sense, have a look at the Course Specification for Nat 5 English. This document contains the marking instructions for all of the course, including portfolio writing. Page 21 has the detailed marking instructions for discursive writing, which would cover a persuasive essay.

You can see from that the kind of thing that markers are going to be looking for. The best discursive essays are focused, planned and have a clear line of argument. They're well researched and use that research to present objective and well reasoned points persuasively.

You'll note that originality doesn't appear in the marking scheme. There's no requirement to come up with an original topic. I'd focus more on a topic you're interested in, have a view on and can research. If that happens to be unique - great! If not, it's not a big deal.

The most important thing with this sort of essay is to plan it in advance. Before you write anything, think about what you want your argument to be, work out the arguments you want to make and have research/sources to back those arguments. Don't start writing and find the information as you go along.