If an ISA change does occur I'd rather see it be to ARM as it is a more broadly licensed archetecture (and its not X86).
BUT ARM just isn't powerful enough (yet).
As do I, tho checking crossplatform benchmarks of CPU/Memory performance a OMAP 4430 @ 1000MHz (2 cores) preforms as a Mac mini 1,42GHz and the Exynos 4412 1600 MHz (4 cores) preforms as the PowerPC G5 (970FX) 2300 MHz (2 cores) and the ARM boards consumes ****loads less power and are small and the G5 power horses wight like 50lb (22KGs).
So they are getting there and fast, a generation or two I can see ARM boards out preforming the PowerPC G5 (970MP) 2500 MHz (4 cores).
Now there are more factors to consider then these tests but overall ARM are not as far behind the top PPC's as one might think in some parts.
Tho im not saying that MorphOS should go ARM, but if they should migrate I would think ARM would be the best bet, and they are not as weak as people might think they are starting to get horse power under the hood and they are still growing and will for a long time and it feels more viable in the long run, besides old Mac hardware there is not much to see in the future of PPC hardware at acceptable price ranges while SoC's will be cheep and easy to get.
Don't get me wrong, I love my Mac mini and MorphOS and I would support them if they stay at PPC, tho I think it's interesting to think about MorphOS and ARM.
Edit: Do not kill me, im just speaking my mind.