Offering up my help (and two cents) as well.
I think Barry and Ian are off to a great start, asking questions before thinking about the solutions. Managing scale can be complex and static site generators are a great solution for some issues of scale, but is scale the first order of business? (i.e what is the actual motivation behind leaving WordPress?) I used to run front-end for EW.com which ran largely on WP and, I imagine, handled scale orders of magnitude greater. If scale is the issue, I would suggest looking at other options before committing to a re-platform, especially if it would interrupt an existent editorial workflow. You can always throw machines at the problem. ;)
As for transparency, a static site hosted on GitHub will provide visible content and records of discussions and changes. However, as mentioned, it would require slightly more savvy from its editors. Many static site generators (I've used Harp and Assemble for client projects), manage content via Markdown and metadata via JSON files. While Markdown can be taught easily and MD text editors can provide assistance, managing JSON structures introduces a bit of brittleness in the hands of a (potentially) non-savvy staff.
Now, if WordPress is not satisfying your current editorial needs, look for plugins and start to search for alternative CMSs. Note that static sites lack many of the conveniences of a modern CMS. One of which that springs to mind is timed publication. Unless you don't mind someone manning the big red release button at odd hours, this feature can be useful for getting content out in time for the daily news cycle.
Anyway, I think a discussion of strategic goals resulting in a set of prioritized features/requirements that can be divvied up and developed should come first.
So, what do you want to build?
Cheers,
furf