Thanks for wanting to contribute. We have a Contributor’s Guide but it sounds like you already know most of it. We’ll also need a CLA, see CONTRIBUTING.md.
As for including third-party libraries, there’s a good discussion here. The Leap Motion license looks good, but at 79KB minified, it is not central enough to Cesium for us to include it in a standard Cesium build so I agree with your plugin approach. Leaflet has done this with great success.
I could see us doing something like adding a new Plugins directory to the project’s root directory and having build options to include various plugins in Cesium.js. AMD users would already be OK. There are a lot details to consider like does each plugin have its own third-parties directory, where do the examples go, do plugins really live in the main repo, etc. We’re not going to be able to flush this out this month since several of us are helping out with NORAD Tracks Santa again.
I suggest just adding support for Leap Motion as part of core Cesium and opening a pull request into the leapmotion branch, instead of master, which will allow us to review and merge your code without slowing you down while we work out the other details. You’ll also want to key an eye on pull requests #1331, which may affect you.