If you want contribute to JWebAssembly then you can:
Post a recommendation or comments on existing recommendation. Such recommendations can to the API or to a runtime library.
We need a large infrastructure on tools that based on JWebAssembly. We will happy to support you. For example with specific features. Inform us if you have start and we can link your project.
Currently it is a little difficult to test JWebAssembly because the missing infrastructure. If you find problems, we will appreciate it if you post a bug report.