The Tree of Knowledge is, of course, another example of the use of models to either make a point, as I am trying to do here, or embody our knowledge. And it is on this sacred tree, which surely grows in the very center of the Garden of Rhetoric, that the rhetor can find, scattered along the different branches of knowledge, the fruits of our intellectual endeavors that compose the many different topics of interest.
All the same, if any useful advice can be given for choosing a topic, it would probably be "write what you feel"; for, undeniably, one of the most important ingredients in a good argument is passion for the topic. And it often seems that the most persuasive speakers are those who are driven, if only because then its almost as if they don't have to choose what they're going to say; they are carried to it, as it were, by the momentum of their emotions. Without this momentum, the natural inertia of a heavy topic causes the mind to drag and stall. And as every writer knows, it's extremely difficult, almost painful, to invent persuasive arguments for a topic for which we have no emotional bias. Any student who has ever taken a composition class simply because it was required knows just how difficult it is to force oneself to write; and often how fruitless.
So ultimately, whatever you choose for a topic, you should make that topic your own. Eat, drink, and sleep it—in a word, live it. Assimilate it, make it of a piece with your current knowledge. Then it will come out in the things you say, almost without effort. That's what we mean when we say "write what you know", almost as if this cliche knows the etymological connection between 'know' and 'kin'. Pick a topic that is, as it were, close to you or to which you can become close, and then you will learn what you need to know about the subject; you will feel it in your soul and burn with a passion when you plead it; and your sincerity will be perfectly evident to your audience. Given that, the rest is mere mechanics. When you know what kind of garden you want to grow, when you've designed it from the heart, and when you're excited about the beauty that it will exhibit, then you pick up your tools and you start digging.