I like your idea (and could also imagine using the idea for my own wiki) and I've already created a feature request in the issue tracker of the include plugin, it can be found
here. I don't have any time currently but I hope I'll find some time to implement this during the summer and I think this shouldn't be too complicated as the include plugin does already support including specific sections of a page and all pages in a namespace, the only part that is really missing is the option that only pages are included which actually have the specified section (currently it simply includes all sections in this case) and that this condition is tested in an efficient way.
I'm not yet sure how these includes could be cached efficiently (or better: how I could check if there is a new todo section in some page), maybe I'll just let the cache depend on any changes in the wiki for this type of include in order to avoid expensive checks if the cache is still valid. If you want to include a large number of pages in the search of which only few actually contain the section I could also implement that all headings are indexed so the include plugin would be able to retrieve a list of all pages which include a certain heading without iterating through all pages and loading the toc for them.
I'm wondering where I should limit the features that should be supported by the include plugin, but in fact it would be possible to search pages by data from the data plugin as it is already possible with the tag plugin.