All good points.
But consider this: in this job, you'll have to be able to persuade people to like the thing you're passionate about. You're fighting an uphill battle if it's something "niche," like Wing Chun. Really, how many average people have heard of it? How many average people are into martial arts? How many people might be turned off by the idea of combat? You don't want to make this harder than it has to be, so I wouldn't recommend WC as a topic, no matter how great we think it is. And it is great.
With WC as a topic, although there's a fitness angle, the "obscure, Chinese and martial" (to them) angle might work against the fitness appeal. It would stand out. And trust me, in North America, people aren't interested in Chinese topics unless they've already investigated them. Or they're Chinese. You get polite interest only or glassy eyes. And then there's Mook's angle on being known as the martial guy, of course.
You need to be able to connect with the customers. You need to speak their language. You need to be someone who can connect with anybody (or just about anybody). You're going to have to be ready, once you get the job, to talk to customers about the weather, sports, kids, cooking, fitness ... all sorts of stuff that might interest the customer ... to get on their good side. I recommend you pick a topic that you can talk passionately about and convince someone else to like because they already like something about it. (Note my choice of words: it doesn't have to be the thing you're most passionate about.)
If I were interviewing you, I'd gauge your skills on how well you sold me on your topic. To do that, you'd have to consider me as an average person, but one who also has interests that align with the company. There's a clue, actually. That person likes what the company likes.
So. What else you got?