It all depends on how he tries to evolve the art. Wing Chun is a principle based system, not technique based. If someone is able to add things while keeping the principles, then I'd say he'd be the better teacher. Also he should take time and really learn it properly. But if he's the kind of guy who dabbles here and there and never really learns anything in depth properly, then he would not be very good. For example, I know of a Wing Chun sifu who supposedly like to combine Muay Thai with Wing Chun. Well his Muay Thai is garbage. His roundhouse kicks are horrible as are his other techniques. His Wing Chun is decent. But if you're going to add other stuff to your art and especially if you intend to teach it to others, you'd better make sure that you learn it properly first. It's like watching most JKD guys do boxing drills. Go to a boxing gym and show them that stuff and they'll laugh you out the door. Some JKD guys are good. Most of them aren't. If you want to cross train and learn other arts, fine. Learn them properly from a qualified teacher. Then you can add things to Wing Chun as your base art. But if you watch a couple of videos or attend a seminar or two and then feel qualified to create your own modified Wing Chun, you'll be in for a rude awakening.