How about a detachable cord? That's a very good idea for another reason too, because the Sensor Bar's cord is very thin and easily ripped/cut/torn/etc. Being detachable means that it could also be replaceable with a new cord, without having to throw out the whole sensor bar.
I also liked Mop It Up's suggestion of a battery compartment, so you could use the cord if you wanted to but if you wanted it wireless you could just detach said cord and pop in a couple batteries instead.
I think those are fine for a 3rd-party sensor bar, and in fact I think there is a 3rd-party sensor bar like this. But they both have the disadvantage of increasing the size of the bar. Battery power alone will probably double or even triple the height of the bar if they use AAA or AA batteries and increase the depth by 50% if they use AAs. It would probably also increase the weight several times at least. At least from holding them to me a single AA battery weighs more than the entire Wii sensor bar.
Making the cord retractable without getting snags would probably increase the depth and height by at least 50%.
Having a detachable cable would make the guides for the wire much harder to implement, and if combined with batteries would require the plug being on either the left or right end of the bar instead of the middle (or there would need to be 2 separate battery compartments, which I doubt they would do for the sake of simplicity and manufacturing costs). The guides could be added back, but this would once again increase the size of the sensor bar.
So I think those are useful features for a third-party or "premium" Nintendo sensor bar, but I think the bundled sensor bar should stay as simple, small, and lightweight as possible.
In fact, I think the next version could be made substantially smaller. The area in the middle, for instance, should be mostly wasted space. There may be one or two simple timing circuits in there, but those shouldn't be more than 1/2 x 1/2 x 1/4 inch (that is the size of much more complicated off-the-shelf timing circuits). Making that area thinner and just have bumps on the ends for the LEDs and timing circuits would be better. Also, making the LED areas smaller should be possible, reducing the height to probably half its current height for large LEDs, less for smaller ones. Also, having the whole thing made out of two or three (maybe even just one) molded pieces of plastic instead of the 5 pieces it currently is should cut down costs.