Bunsen
Admin-Witchfinder-General
Port NextSTEP/OpenStep/GNUstep?Something Unix-like (i.e. solid "modern" OS) ... Kind of like what OS X would have been if they'd gotten started 10 years earlier ...
http://en.wikipedia.org/wiki/Nextstep
http://en.wikipedia.org/wiki/GNUstep
http://en.wikipedia.org/wiki/OpenStep
http://www.nextarchive.net/
Ok, so not exactly "classic Mac app compatible", but with Objective C onboard, you could code apps that can be compiled to run on either 68k machines or under OS X