I'd like to start off by saying, could the nunchuck house the MotionPlus?
But let me digress....If I'm assuming correctly, MotionPlus requires calibration at the beginning of the game in order for it to know where and how you are manipulating the controller in space. It's because of this calibration that it knows where you are pointing even if it's off screen. Now stay with me for this next part. If it knows where your pointing off screen, then it can use that information accordingly in game. That is nutz and let me tell you why.
In a FPS, if they can figure out how to move and look around without the need for the wiimote, the possibilities for the genre EXPLODE. I remember waaaaaay back when I wrote a piece about how the revolution controller would work, describing the FPS genre in detail. One of the things I mentioned was the ability to shoot behind you. Now, that might actually be possible! But before I get into that, let me explain the setup.
NOTE: I'm assuming the nunchuck would need a motionplus upgrade for any of this to work, unless someone more clever than I can figure out an alternative setup.
Imagine the nunchuck controlling your "body" (both your movement and your view), and the wiimote controlling everything else. You would walk by using the analog stick and move your view around by tilting the nunchuck. Looking up would require you to tilt the nunchuck's nose up, similar to a plane. To look to the left or right, you would rotate the nunchuck on its side. The sensitivity could be changed to allow for slower or faster turning, but regardless, very subtle movement of the nunchuck is all that is needed to move the camera. These two control methods would result in circle strafing that would consist of tilting the nunchuck in one direction and holding the analog stick in the opposite direction. As crude and as uncomfortable as that may sound, it frees up the wiimote for possibilities that just weren't achievable before.
We all love the ease of aiming that only the wiimote can provide, but we dislike the fact that our aiming controls the camera in tandem. The idea of being able to shoot anywhere on the screen like a light gun intrigues us, but having a bounding box deliberately restricts us. With some clever programming and game and level designs, one could make an engaging experience regardless of the restrictions (MP3); but the fact still stands, at it's best, the wiimote acts more like a trackball than a pointing device. With this setup I am describing, the wiimote will act more like wand than a pointing device. The difference is huge. The reason being that the wiimote is finally detached from your view.
Now, imagine walking around in the game world with your nunchuck, looking at different objects high and low, left and right. You only have one item; a gun. With your wiimote controlling your gun 1:1, you have the ability to twist and turn, tilt and point, and rotate your gun however you please. And since your gun is independent of your view, you can manipulate and shoot the gun off screen. That means, I could literally shoot behind me or even shoot myself. But let me back track. When your pointing the wiimote at the screen, you will see your hand and your gun. You will have the ability to see exactly where you aiming, and more importantly, you have the ability to shoot anywhere on the screen. I hope I explained this well enough because I'd like to describe exactly why this is so monumental.
There are many reasons why this would change the FPS forever. For one, you could control anything 1:1 without disrupting your view. You could use your items in a 3D space without being restricted by your "window" or view. That means you could literally shoot blindly around corners, above crates and even behind you.
Interestingly enough, you could attach an item to your nunchuck, such as a flashlight or another gun. Since you are controlling the view anyway, it would work the exact same way it always has; with your aim attached to the center of the screen. So exploring that decrepit moonlit farm with your flashlight in one hand (nunchuck) and your gun in the other (wiimote) would be all the more engaging. The possibilities are seemingly endless, especially when you start thinking about other genres.