After some research I settled on attempting a rewrite of KoaJS. The 3.0 roadmap of the project indicated some interest of potentially rewriting the project with TypeScript from even a couple years back. This interested me as it could provide an opportunity to give back to a project I have used rather extensively and allowed for me to focus purely on learning TypeScript due to my familiarity with Koa.
After chipping away at this effort on and off for a few weeks I reached a reasonable stopping point. I rewrote the implementation in TypeScript and ported the tests from mocha to jest.
Overall, I found the experience rather pleasant when writing TypeScript. The use of types greatly added to the clarity of the code, eliminated ambiguity, and created safer code with guarded type checks. This resulted in a Koa implementation that I would argue is easier to maintain, is more resilient to misuse of consumers by inappropriately utilizing the API, and I even believe I caught a few bugs along the way.
Unfortunately, after presenting my side project to the Koa community via a GitHub issue, the maintainer of the project did not find a TypeScript rewrite as something beneficial and closed my issue. So, I will shelve this for now, but at least it served as great learning experience for working with TypeScript on a reasonably sized project from start to finish.
You can find koa-ts at: https://github.com/kubejm/koa-ts