AGA implemented on FPGA is science fiction at this point - one is better off with UAE.
Isn't that in essence what the Indivision AGA is? An FPGA Lisa chip? I guess we'd still need a replacement Alice though.
As for the CPU, it doesn't need to be softcore, Freescale still make '040 and '060 CPUs.
Either way, it's still just a pipe dream. :-)