Depends what your goal is as well, I've no interest in creating a startup with the potential to be a huge company, 1) those ideas are really hard to come by 2) they will inevitably attract competition 3) unless they are not trivial well funded competition can and probably will catch you rapidly 4) first mover advantage works on a product that has network effects but not much else.

The product I want to build next is a niche product for a very specific (but quite large in absolute) terms demographic, profit per user (after tax/VAT/Hosting minus staff) is approx $6 per month, at 500 users I'd be doubling my current income, at a 1000 I'd be able to work on it full time, at 2000 I'd be very comfortable, anymore than that would just be gravy.

It's not even a difficult build (I've built much more complex projects for clients) but the market doesn't exist at all (which means it's either a really good idea (unlikely) or a terrible idea (more likely)).

In either case I'm gonna give it a shot since it's a product I'd have bought if it existed immediately and happily.

Why not start by building a landing page and seeing if you can get at least a few hundred people to sign up? Seems like a much easier way to determine if the market's there than diving in head first.

Honestly? Because if I can't sell it, I'll give it away, its something I badly wish existed for my mum over the last few years so if I can't make a living on it I'll host it for free and fund through my current business as a donation.

True, lots of great ideas look like bad ideas at first (again, pg). But there's things that look bad because they're bizarre (hey, let's rent out bedrooms!), and things that look bad because they're trivial (hey, let's help people find the nearest bar!).

The good bizarre ones could wipe out an existing industry. That's what makes them interesting.