Replace the scroll wheel with a smallish trackball. Treat the vertical axis like a normal scroll wheel. Treat the horizontal axis like a second horizontal scroll wheel. Yes, I know they make mice where you can press the wheel to one side or the other to imitate a second scroll wheel, but those sort of suck, it's more like combining a scroll wheel and a nubby mouse instead of combining two scroll wheels.
Other idea: A mouse that works more like a joystick - pressing the mouse to one side is the same as continuously moving a normal mouse sideways. I suppose it'd be sort of like an inverted nubby mouse, except the nub is alightly longer and attached to a base (with some form of ball-and-socket connector?) so it doesn't slip, and it'd look like a regular mouse stuck to a big base. Something like this might be nice for shooting games, actually.
I'd also settle for the original rectangular Logitech Trackman being rereleased except with optical guts and maybe a scroll wheel on it somewhere, perhaps on the middle button. But that obviously isn't going to happen. (Although perhaps I could take an old one and stuff the insides from a newer mouse into it, that's almost tempting...)