John Jantsch in How to Find Killer Content Ideas references some suggestions to solve the question: “What should I write about?” if you are seeking to design your blog/website and social media content to attract business traffic/conversions.
His first two suggestions, BuzzSumo and Keyword Tool, require significant subscription fees, that is if you consider “significant” to be much more than “free”. Of course both services allow for some exploration for free, and a start-up trial subscription offer.
i won’t rush to sign up for these services as I’m an extreme Internet cheapskate, but resources like these may be worthwhile if you are serious about maxing out your content marketing/SEO effectiveness, or more obviously, if you provide supporting/marketing services for a diversity of clients and so won’t easily know what works best for your specific site/niche.