Besided wanting an OS X version, I would love to see additional video chip sets emulated. Of all of the operating systems in my collection, the only i386 OS I can't run is Be!
Also I would like to see the different environments stay active as you move from one to the other, and all of them to be able to see each other on the network. I can just image setting up a virtual network with an NT4 Server, NT4 Workstation and 2000 Pro all inside of my Mac (I can run them all now, but they are paused when you are using the other environments)! That would stop most PC users in their tracks to see something like that! Actually that was the only reason I upgraded to 4, 3 was fine, but I thought you could run multiple environments at once (all actually running) in 4.