So you need an idea on how to come up with an idea? For most people, it's either intuitive or from experience. The idea is not everything, you can't be a professional footballer and have an idea to design a car like Tesla. I look at the existing projects, find their shortcomings and figure out ways to fix them. Or use the existing crypto services and think about how the experience can be made simpler. For instance, I want to create a payment gateway that is way better than BitPay and doesn't ask for KYC.
From my experience these are exactly the things that doesn't really work. If you run a business in established market, you'll most likely fail regardless of whether the competition sucks or not, they were first. That's the most important thing. The only reliable business is to be first to provide some service or sell some product. The problem with a new idea evaluation is that you never know the reason why it doesn't already exist. Is it because it's a genius idea and only you can provide it, or is it because there's no demant for it.
You mean if you create a product or a service that doesn't exist already has much higher chances of success? I don't think so. Have you watched the Shark-Tank? They get new ideas and products every week but how many of those actually hit the market? Also, with that logic, people would have stopped creating gambling sites or crypto wallets since there are too many of them and the market is very established? There is UBER but new Taxi apps continue launching. It doesn't work like that. If you see shortcomings, you can provide something better alternative over the existing products, people will slowly adapt to the new service. There was no "demand" for facebook like website, Zukerberg created that demand. Look, how many social networks you see around now!