> IIRC AmiKit X opens a minimal Linux setup and then runs AmiKit X

Not quite. AmiKit ('X' from 2017-2019 just means v10, 'XE' from 2019-2022 means v11, current since 2023 is v12) requires Windows, Linux or macOS pre-installed. It simply installs on top of those. The Rabbit Hole feature launches native programs of the host OS, so it's Linux programs only when installed on Linux.
Generally, the notion that something starts something else and only then runs itself makes hardly sense.

> running MorphOS first precludes Linux or Mac OSX already
> being open as MorphOS is the OS that is actively running,
> but if there was a way to emulate those systems [...]

A more realistic idea compared to somehow running Linux or Mac OS X from within native MorphOS is the other way round, like AmiKit and numerous other solutions do using UAE, i.e. running QEMU + MorphOS on top of pre-installed Windows, Linux or macOS on a modern x64 or ARM system. This has been possible for years already. What's yet to do is implement the Rabbit Hole feature (to not require actively switching from the QEMU context) and make it possible to register MorphOS running on QEMU for the longer-than-30-minutes MorphOS experience.