The best way you can be able to come up with a good business idea he is you making research with research you can be able to know the kind of business you want to invest in.To my descretion. I think brainstorming on existing business could be okay. But try figuring out the usual or normal way of the business model then try attaching a unique system to the existing ways of running. That could make it stand out from other vendors running it. Uniqueness and Branding is the key success to every existing business.