The K-Machine would be a super interesting retro-computing project to get going. The schematics, the Lisp Machine system associated for it, are available (I'm unsure of legal status). If anyone is interested in driving that, I'd be really interested to help (but my primary interest is just the CADR due to lack of hours in the day :-) )

Here is a wonderful article about the K-Machine: http://fare.tunes.org/tmp/emergent/kmachine.htm