The problem I have with these kinds of ideas is that you'll end up with a 'modern' computer with a very slow old computer attached to it 
The main appeal of Classic hardware to most people is in running the original 68k games/demos/apps and a modern expansion doesn't fit into this appeal.
I'd personally draw the line at a really fast accelerator with an on-board HD controller but that's just my personal opinion. I think as soon as you replace the original chipset (eg AGA) then it becomes pointless (see above).
I'd like to see a new cpu/graphics card combo for the A1200 trapdoor slot. How rare are those blizzard ppc's with blizzard vision? Far too rare in short.
Even an 030 card with RTG would be nice...
Suppose you could argue that there's big-box Amiga's for that sort of thing, but I'm sure a card like that would sell like hot cakes.
