There is (fake?) MUI5 that should have sources
or MUI5 m68k OS3 could be fixed to work on AROS?
https://github.com/amiga-mui/muidev/releases
https://github.com/amiga-mui

MorphOS is progressing, about the speed of AROS, but unity
of this could help. However, its chicken and egg >:D
I guess for MorphOS team ability to license and integrate maybe
some parts of AROS PPC recent could be interesting, but not what are you proposing.

However, seems Amibient GUI is floss and if MUi port could be done,
could make AROS look like MorphOS.