Well, we need to use Windows audio drivers for hardware so need to support a big chunk of the Windows kernel. And use existing Windows plugins, so better support the Windows runtime stack and libraries and the Windows GUI. And we may need to exchange files with other people or connect to websites for plugin authentication, so better have a pretty full network stack, and support for Windows network card/wifi drivers.
I suppose it doesn't need to run Office or Notepad, but apart from that, you're basically looking at replicating...Windows!