More importantly, it makes it too expensive.
The very first thing we need to look at in any concept is the cost per unit, I think. 68K CPUs cost about $100 each depending on the model. If you can live without the MMU and at low speeds, they get cheaper.
68060 and 68040 are also too expensive.
Coldfire chips are cheap enough (~$30) but I see your post about them lacking an FPU. So, we either live without an FPU in accelerated systems, or....
If we're going to try to implement an FPU in an FPGA, then I'd say we're better off just going all the way back to the emulate the 68030 in an FPGA. Besides, the CF doesn't have a built-in interface to an FPU the way the 68030 does, does it? That would make providing an external FPU much more daunting.
FPGAs with about twice as much logic as found in an actual 68030 cost under $20 and run at 200 MHz.
The 68030 contains about 273,000 transistors. The 68040 contains just under 1,200,000 transistors.
If we assume that there are about 4 transistors per gate (this is very conservative from the point of view of this estimate) then the 68030 contains about 70,000 gates and the 68040 contains about 300,000 gates.
The Xilinx XC3S500E contains about 500,000 gates and costs $20.75 when purchased individually from Digi-Key. I'm not sure how FPGA gates translates to CPU transistors or even CPU gates though.
There are at least 4 transistors in each real world gate, except inverters which only have two. If we assume an average of four (which should be low) we get the numbers above for total gates in the target CPUs. I would assume that one needs many more gates in an FPGA to get the same functionality found in a CPU, because we're not custom designing the logic. But how much more?
The XC3S500E has 7 times as many gates as a 68030. Is that enough? Is it too much?
Does a 200MHz FPGA run non-optimized logic at a speed faster than 40MHz 68030 with custom logic.