While you are building for a niche right now (you should be at least, since trying to appeal to everybody and trying to do everything at once will not only make you lose focus but will also make your early adopters confused), you might have a grand vision.
This is actually one of investors' favorite questions. The reason is because usually lean startups start out tackling a problem for a niche market while holding off on the bigger vision. This is not to say you should start building for the "bigger" problem. Because as a Lean Startup, you need to satisfy your niche first before you can move on to the bigger picture - so you can cross the chasm - move from early adopters to the majority market.
Case Study:
Digg's first version of the product only catered to tech news and thus to the techies. However, they later "zoomed-out" after they were more popular and became a social news platform for every topic, which made Digg then a platform for everyone. The thing to remember is that if they had not started with the tech news sector and tried to target everyone, they might not have been able to get enough users to even become a viable business. The trick here is to be focused in the beginning, get 1-3 demographics first to solidify your user base first, then expand to get to more demographics after you reach a certain size.
