A good opener, first and foremost, fails to repel. Right? So it's interesting and engaging. It lays out the terms of the argument, and, in my opinion, should also in some way imply the stakes. Right? Not only am I right, but in any piece of writing there's a tertiary argument: why should you spend your time writing this? right? "so here's why the following issue might be important, useful, practical." I would think that if one did it deftly, one could in a one-paragraph opening grab the reader, state the terms of the argument, and state the motivation for the argument. I imagine most good argumentative stuff that I've read, you could boil that down to the opener.