After combing through the cobwebs of my mind, I thought about an old TRS-80 game called Santa Paravia. I'd always liked "Hammurabi games," the spiritual predecessors of games like Civilization. I figured it'd be a relatively easy port but, presuming I put some effort into building a modern interface, not embarrassingly trivial. I also figured it was far too obscure a game for anyone to have remembered.
The first Google response on a search for "Santa Paravia" is a history page with a Windows port in Visual Basic.
Trying to get more information on this led me into the world of TRS-80 emulators, something I seem to bumble into every couple of years. There's a Model I emulator for "classic" Mac OS, but for Unix there's xtrs, a frighteningly complete Model I/III/4 emulator (down to emulating peripherals and add-ins like hard drives, graphics cards, mouse support, and a collection of utilities to manipulate virtual disks from both Unix and TRSDOS command lines). A bit of hacking around got xtrs compiled to use Apple's X11. Finally, a use for X11--emulating even older machines!
Of course, I suppose another exercise could be porting xtrs to use Cocoa natively. Ack.