AROS -- an AmigaOS-like environment -- can run in "hosted mode" on Linux or Windows environments, maybe Mac as well, in addition to standalone.

https://www.aros.org

Note that because it's an API-compatible reimplementation of AmigaOS, it's not really a "fantasy OS". Hosted mode is rather a tool for serious OS development. Nevertheless, it's much more like a modernized retro Amiga than it is any extant DE for Windows/Linux/Mac.