I have a site with 10s of millions of pages. About 10% of those are indexed by Google. The less lucrative are robots.txt blocked from crawling, which considerably increased traffic and ad revenue.

I'm planning a big update soon that will add a new section to the site, with about 6 million new pages.

Since I know that Google is not going to index any more pages, I have been wondering if it would be best to put this new section on a new domain or subdomain.

My understanding is that if I create a subdomain on the existing domain that some/all of the authority from the parent domain will flow into the subdomain. Is this correct? But what about indexing? Will Google be prepared to index potentially 6 million pages on this subdomain, and leave the 6 million indexed on the parent domain?

My guess is the answer to the second question above is - no. Which leaves the option of a new of purchased domain. This makes some sense, because there is some considerable difference between the topics of my old and planned new content. For the new content there is already a competitor who had multiple sites, inter-linked. The downside here is I will be splitting authority and have to build up authority on a new domain. But this seems to be the most likely way I can get lots of pages of the new content indexed.