SEO has evolved over time and is not the same anymore. There are various complications and surely you would be addressing various issues while contemplating your SEO strategy.
Sub-domains VS Subfolders
One of the major challenges faced over a while are whether to place your content on subdomains or subfolders? Though there have been many versions from Google saying they have become smart enough in associating content on the subdomain and the main domain.
But there have been certain doubts over this issue. Companies have shifted their content from the main domain to a sub domain but the results for them have not been very encouraging. In fact, their Google analytics showed a major downward graph as far as the traffic is concerned. So the big question remains, where do you actually place your content?
Since Google is still not perfect, one should always keep their content on one single sub and root domain, preferably in sub folders. Following this strategy will help you benefit in your SEO strategy to the maximum.
Content and Link Optimization
The common perception is that links is good for SEO and the more links and link diversity you have, the better it is. You can actually have all the links from different pages and add them to your main site benefiting your ranking in a major way.
But, this is not true. By getting the links through your subsite to your main site, you are actually creating a barrier for the links to transfer fully, thus losing on the full link equity for brand, user and usage data signals and any potential social signals. Now, to get the maximum possible benefit, collect all the ranking signals on one sub and root domain. In a case where you have multiple sites, 301 redirection of all these and having one main site is the best strategy that one can follow. SEO services always need to be in sync with the updates by the search engines to gain maximum benefit.