You could do a speech debating whether or not the entire "Everybody holds a 15 minute speech" idea is a waste of the students' time. When the speakers can choose any subject they want, it most likely won't be anything educational. Holding a speech for 15 minutes is good, and educational, but is it really worth each and every one of the students having to sit and listen to trivial things for 450 minutes (presuming there are 30 people in your class)?
That's probably something along the lines of what I would do, at least.
You might not be the kind of person who does that kind of thing, though. Whether you agree that it's a bit of a waste of time or not will tell you something, though. If you think that it's a waste of time, you should talk about something educational/the listeners will find very interesting to make it not be a waste of time for them. The problem with doing something video-game related is that over half the class most likely won't care, and it most likely won't be that interesting to the ones who do, as you have to do it at a simple level. You should try to make it something they care about, rather than just something you care about.
If you don't think it's a waste of time (and that presenting a 15 minute presentation is worth 450 minutes of listening), I can't really help that much. It doesn't narrow the field down at all, because it's just an excercise for you, and not for the listeners. Actually, if you think it's just an excercise for you, you should make it about something you'll struggle a bit to hold a presentation about. Like, a poem if you're a maths/economics/physics/etc kind of guy, or a physics presentation if you're an art person, as that's the way you will learn the most.
I kind of lost my point in all that. Here's the deal: You have to decide if you're holding the presentation for yourself, or for the audience.
If it's for the audience, talk about something you think they'll all be interested in, and have learned from.
If it's for yourself, talk about something you're not an expert in, as that's the way you'll pressure yourself the most.
Hopefully that narrows it down a bit.
