One needs only to look at this thread Is there interest in supporting a Metal version of Cycles for MacOS?
to see that Metal is a long way off at best. I wouldn’t hold my breath for it.
On the other hand, I think an iPad/Tablet version of Blender isn’t a bad idea… maybe the idea could be developed with Windows Surface tablets for now, to get the UI/UX working while support for other platforms is considered/developed? Or maybe jail-broken Android devices running some kind of GNU/Linux system? Maybe Android itself is a possibility.