Personally, I think buying domains that seem good, and then brainstorming ideas for them is risky. You're more likely to end up with a questionable foundation ("Maybe it could be a site about dogs with funny socks?") or something outside your area of interest/expertise. IME, it's better to set your idea and then find the domain.

By all means, speculate on domains and collect a few strong ones (I do), but I'd be wary of picking ideas based on the domains you can grab.

You used to be able to make easy money from SEO-oriented sites based on keyphrases, but that's dried up a lot and content/concept is king again.