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There is flinux from wishstudio on Github for a fully-in-userspace layer that works in Windows 7, it's unmaintained now though

And there was also CoLinux, a linux kernel on top of windows, supersweet. http://www.colinux.org/

And there was LINE.

Isn't the license concern pretty major though? IIRC even in the current form Wine at the early age had been quite controversial, due to legal concerns. They consciously prevented those who have knowledge of Windows internals from contributing to it just to make sure it's a clean room implementation.

IANAL, but doesn't OpenJDK basically do the same (implement documented API interfaces) with Sun's Java? And the Windows syscalls are IIRC also publically documented...

OpenJDK was/is an official Sun (now Oracle) project.

But others did reimplement Java without authorization, such as Apache Harmony, and, well, I think you need to look up Oracle v. Google.

Some WSL components must run in supervisor mode, so a translation layer like Wine or WSL is not powerful enough to emulate it.