How Do You Keep A Primary Domain Active When Publishing New Content On Subdomains?

In episode 194 of our weekly Hump Day Hangouts, one participant asked how one can keep the primary domain active if publishing all new content on subdomains alone.

The exact question was:

Hi guys, thanks for the event and the oportunity of asking you.

I have a long term fashion WordPress blog. Last year, I made many local subdomains (city subdomains) so the main blog (primary domain) has no new content, as long as I publish now in each subdomain. What should I do to make it still active?

I think about publishing in the primary domain, using IFTT, all new subdomain articles, with atribution link to the subdomain where the entry belongs originally.

This domain has visitors and it can help everyone to know their local subdomain, but not sure how to do it better or if this is a good enough idea in SEO terms.

What do you think?

Thanks again!