If you're not worried about being compatible with any other particular computer out there, I have a 68k board that I designed some time ago.
1 megabyte of ROM and RAM, 2x UART (TL16C2552 or similar, TTL levels only), a bunch of expansion slots with configurable address ranges, configurable wait states, configurable interrupt auto-vectoring, configurable option to relocate RAM to start at address 0...
It uses a handful of standard logic gates and 3x GALs. The board is about the same size as an A5 sheet of paper.
I've got an 8MHz HC000 running at 16MHz on mine, but a friend has run one up to 32MHz.
I've also been working on a GCC based "tool chain of sorts" to allow me to compile C, and I've built a simple serial based loader (using a python script) so that you don't have to keep popping ROMs to run the next iteration of your code.
Im in the UK so if you were interested I have a couple of spare boards.
Edit: The first expansion board I built for it has an RTC, battery backed RAM, Intel 8254 timers, and a watchdog timer, and have PCBs for this as well.
I started building a VGA card (prototype only) that uses a 128MC CPLD and which does work (provides 80x25 text mode 16 fg/8 bg colours), and had intended to start building an Ethernet card, but stopped there because unfortunately my board doesn't support anything else being the bus master and the Ethernet controller I was intending to use needs to become bus master to send/receive packets.
I've been considering a new design, but this board is still useful.