The wider the topical nature of subpages, the higher level of authority you need to become in order to rank for them all.
You're trying to break from a traditional website "theme" model and become a know-all authority. That requires many many links straight to the homepage, and many many deeplinks into the content pages. You need everyone in each sub-topic citating your subpage as the authority in that niche.
Wikipedia pulled it off - see that for an example of this in pratice.
If you have a decent internal link structure and navigation, you'll get all pages indexed. But you're only likely to rank well on one or two topics (that'll vary depending on how competitive each keyword term is) without having a massive inbound link structure out there.
Any decent search algorithm will ultimately theme your pages collectively. They will not assume that you're an authority on everything. That may not necessarily be deliberate, it's a result of the interlinked structure of the web. If you visualise the web as a collection of topics, and think about which sectors your links come from (particularly the high quality ones), you'll picture how that bias exists. Link structure is 99% of it, in particular, deeplinks from other authorities in each keyword niche.
I have heard experience from several webmasters that a subdomain per topic works better (at the moment at least) than a sub-directory per topic.
If your pages are actually all related to a parent topic, concentrate on the parent topic and your internal link structure. If you already rank top 10 for "widgets", it's far easier to get a page top 10 for "widgets reviews" or "silver widgets". You still need deeplinks from authorities to the "widgets reviews" and "silver widgets" pages, but you're already halfway there with your theming.