@DavidF
MUI is free for MorphOS.
Zune (MUI-clone) is open-source, and should compile on all OSes if needed.
OS4 will come with an unregistered copy of MUI, that will allow you to run MUI-apps (no need to buy a MUI-key for that),
but won't allow you to change the standart-look.
Also a GUI-system is needed, and GadTools/Boopsi on there own are just not good enough.
ReAction, Triton, StormWizard and other are also no help, as they are all closed-source, non-free and most aren't
developed anymore (or just for one OS).
Porting QT/GTK or so may be an idea, but would that still feel "amiga" ?