The "what should I bring?" question...?

You can avoid this by firmly sticking to the 'don't bring anything' idea. Or you can go full on pot luck. BUT, that means there might be similar dishes. One thing that I've had friends do is give poeple general guideline. Ask one to bring a fruit dish, their choice of what it specifically is. Ask another to bring a dessert. Another to bring a vegetable of some sort. That way, if they insist on bringing something, you have an idea of what to plan around.