MIPS, Coldfire, ARM etc.. looks nice. What pesters me most about x86 is the total lack of good engineering.
Let's stay on big-endian with linear addressing, besides that cpu doesn't matter too much. Just the manufacturer have a good feature plan. Don't forget that ARM definitely use their legal department!, which may spell problems in the feature for any FPGA re-implementations like minimig.
Coldfire has one advantage, it's supposedly similar in instruction set to m68k so a port can be easy.
What AmigaOS has over other OS is Speed! and small resource need. Let's compete with the strong sides, and not bother to compete where AmigaOS has it's weaknesses.