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Great advice.  Thank you very much for helping me think this through.

Thanks very much.  Appreciate your detailed and documented response.

Is the directory related to the theme of the site? So for instance, if you have a site that covers Facebook business marketing and you add a directory of consultants who offer Facebook marketing help, then you're sticking to the overall theme. Likewise if the theme of the site is community journalism, separate subdomains that cover (or are directories) a variety of topics stick to the overall theme. So assuming your idea is a benefit to your site users, a directory will help your SEO because inbound links will contribute to the overall root domain authority and outbound links will establish yourself as a trusted hub of information.