There's no reason why a 68000 series compatible CPU couldn't be made to run at GHz, except there's no market for one.
As Toni mentioned, the chipset trundles along at the same old comparatively measly speed. Even with fast enough RAM on the CPU board, the chipset's design really needs to be updated so that it still doesn't hog near 100% DMA time when the elaborately computed Unlimited Detail 3D Workbench/Game graphics is to be made available for Denise. Adding a simple VDU to the VDU card fixes that, and then suddenly you have not an enhanced Amiga but two computers inside an Amiga case...
I fully support remaking Amiga hardware in new components, and that includes the CPU. Nothing wrong with seeing new Motorola-CPUs making possible new accel cards.
Remaking the chipset is still the hard part and the holy grail to ensure a hardware future.
One way to enhance Amigas way beyond CPU upgrades would be to dev some hardware hack to 1) let CPU and chipset access chipram simultaneously (innovate some dual-bus RAM, or RAM shadow copy) or failing that, 2) some DMA thingy to "copy fast" like on [that] C64 [thingy] (but from fast to chip).