There is already opensource Windows implementation: Wine. Perhaps they could contribute there.
In the meantime Windows 10 certification no longer requires option to disable secure boot. It might not be possible to install alternative operating systems on new computers.
Doesn't really matter to much whether Wine exists or not. People are not arguing about whether Windows software runs on Linux or not. What people actually want (or would like, I guess) is for Windows to be open source. They would like to be able to contribute, maybe, and to avoid closed software.
In the meantime Windows 10 certification no longer requires option to disable secure boot. It might not be possible to install alternative operating systems on new computers.