A MCU that is completely (and more importantly, natively) Mac-supported, I.E. has native Mac simulator, emulator, support for IC debugging, programming in a variety of languages (including possibly graphical flowchart programming). Another idea I had was a MCU programming/eval board that plugs in to an iOS device, and can be graphically programmed/debugged on there.
There is already a wonderful Mac based logic probe, so I'm going to say a USB oscilloscope (a good one!) for the Mac. Should have support for recording long waveforms and signals, as well as saving images and video of them. Fat chance of that happening, unless I do it myself
Liking the idea of commercially available waterproof motors. There aren't nearly as many water based robots as there should be, IMO. I have a few steps to go before I'm at that stage though :|