My current project is a more powerful version of a Z80 based multitasking GPC that I helped a guy I work with design and build. I will also be designing an 8085 version. Both will have support for plug and go (plug and play sounds silly considering the thing is not for playing games) using eight AT-esque expansion slots.
Why use CPU that are so old you ask? Because theyre simple to design systems for and hobbyests still use 8 bit systems quite often.
For a class I worked on a reduced version of the m68k chip - basically fewer registers and only capable of word-size instructions, not byte or long word - but it was still cool.
How are you going about this? Simulation(in that case, what tool?), FPGA(cool), discrete TTL gates(if you're a glutton for punishment) or god forbid actually getting it fabbed?