If AGA compatibility is mandatory it is better continue 68k support in way or another than switch CPU.
Well, the planned approach of "just" a new turboboard for existing Amigas is without doubt the quickest and most compatible solution.
Just for me personally I considered that much money better invested in a chimera of PPC-SOC, Minimig, AGA+ from Natami and the 68k-reimplementation by Tobiflex, because it would probably be more long-living than the ancient Amiga mainboards and would in addition offer nice possibilities to play around, expand and update due to the FPGA.
But, obviously, interconnecting these two worlds would require a hell of development work on the software side as well, so simply book this idea as "just arsing around". :-)