I've been looking at how some of the classic Atari game controllers were wired, and have made a couple of observations (which have doubtless been noted long before, but hey, they're fresh to me):
One pair of paddles and one driving controller can be wired up to the same connector - while the paddles use the two analog lines for rotation, and joystick left/right for fire, the driving controller encodes its rotation on joystick up & down, and maps its fire button to the joystick's fire button. All you'd need is an adapter to let you physically plug them both into the same port. This could be nifty if one wanted to create a 6-player party game with rotary controls, so long as the game's not too worried about whether any given controller has a limited range of motion.
Unless I'm mistaken, Atari 2600 Omega Race is nearly compatible with the Atari Pro-Line joysticks. Omega Race was designed to use the CBS Booster Grip accessory, which fit over the standard Atari 2600 joystick, and provided two extra fire buttons, wired to the analog lines meant for the paddles. As it happens, the Pro-Line joystick also uses the paddle lines to distinguish between left- and right-button presses.
The trouble is that, for compatibility with normal 1-button joysticks, the Pro-Line controller uses the normal button line as the power-source to feed paddle lines, so pressing either button also pulls the Button line to ground. Since Omega Race maps the normal joystick button to a second thrust key, if you were to use a Pro-Line joystick, you would end up with a thrust button and a thrust-and-fire button.
If, for whatever reason, you want to play 2600 Omega Race with a Pro-Line joystick, your best bet is to either re-wire the joystick or whip up an adapter, so that the normal button-line on the joystick's end is connected to the standard +5 line that the paddles normally use. You lose normal 2600-compatibility, though, so you probably don't want to make this a permanent modification.
(That said, Omega Race supposedly also works with the Colecovision controller, and I'm still trying to wrap my head around how that one works from the Atari's perspective.)