There are more than a few of us who have thought about that. Its an expensive proposition and currently its hard to argue against the logic of the developers. G4 and G5 Macs ares cheap and plentiful and will perform as well as anything that could be constructed with currently available PPCs (discounting the idea of building a Power7 based MorphOS system).There are some interesting processors on the horizon. Freescale is about to introduce their e5500 cored P5 processors and Applied Micro is working on "Mamba".

But using these would be a pretty expensive proposition for a small scale run (a few hundred) and ideally whoever developed a board for MorphOS would want to pay for the port (to assure that its worth the time of the developers).

Don't discount it as impossible, but for right now, we're OK with what we've got.