I can honestly say that I would love to see SOFT CPU for an amiga, the ammount geekery I could have would be AWESOME!
however when we start talking level shifters, we need to look at 32bit arrays in the SMD package, probably cheaper than $2 a pop - but you will need a few of these.
this will complicate an alraedy complicated board, I am not saying it cannot be done, I do believe it can - however this would be more of a crusade for a few than a real commercial development for the masses.
Another IDEA would be to look at a Crusoe Transmetta CPU - LOTS of power in the GHz Range and can use a nice 2MB cach for instruction conversion and byte/word swapping.
the Transmetta CPU also has the benefit of being low power, hence passively cooled - but again we are stuck without 5volt conversion or tollerance. =(
I have to admit I think the transmetta could work better than an FPGA, mainly because it could run as two different CPU's at the same time pending on its instruction fetch.