Perhaps a board that can use different versions of Agnus, Paula etc? and existing expansions?
As for expansion bus, I'll vote for PCI33/32 and Zorro3
Sounds good.
Why hasn't this been done before?
With the right '060s we could see as high as 100MHz.
AGA, Buster11, the rest of the latest chips, faster IDE (and the other improvements that could be introduced).
This would way outperform a 4000.
In a modern form factor?
you'll have trouble meeting the demand.
How about something like a catweasel floppy interface and a couple SID chips?