A sport followed by a country: Ugandafootball, englishcricket, dutchbasketball. Could just be a landing page or a redirect. Passive income/traffic.(Blackhat SEO these)
By region and profession : OhioPlumbing, ManchesterHairdressers, parisskydiving.
You could easily research some of the businesses within the area e.g. Ohio and send them email/sales letter telling the benefits on keyword in search engines and how ohioplumbing.com would rank higher than the georgetheplumber.com. Also with this sort of upselling of domains don't forget the potential in the .co.uk/ country ending. manchesterhairdressers.co.uk would be better for them since they are only looking for UK traffic in manchester.
Another way is just having these as a landing page. So when a user clicks ohioplumbing.com it would take the user to georgetheplumber.com. And you would remain the domain name owner but direct traffic to there site for a cost.