At the moment my ideas on all these areas are extremely hand-wavy. Ranging from "you don't really need it," to "somebody else's problem" :)
I feel fairly strongly that I don't want to add very many more layers. Ideally just one more. As more high-level languages get added, I want the lower levels to be integrated into Mu, so that they can in turn become the new HLL. But that's just an aspiration at this point.
One thing somebody else suggested was Plan9's assembly syntax. It may provide some inspiration for making SubX portable.