The wheels are pointed straight ahead, or very nearly so, probably 99+% of the time, so it would be counter-productive to design a "bubble" on the side to accomodate the turning wheel, right?
If something were closer to the wheel, and moved with it (in one way or another), it would "stick out" and increase frontal surface area only when the wheel was turned sharply. This would only happend at low speed when drag ain't a big deal anyway.
OK, here's what I had imagined (actually 2 options):
Option1: Picture a motorcycle front fender. One that extends about 1/2 way down from the top of the wheel. It would be mounted to the wheel spindle behind the wheel, and of course pivot with the wheel. This is probably the easier of the two, but clearance issues inside the wheel well may be an issue.
Option 2: Regarding my "flexible" tease: It consists of 2 parts. Part 1 is a skirt, in the fashion of the rear skirt, except made out of something firm, but rather stretchy if pressed on, such as polyurethane, or some similar polymer. I guess it also could be rigid and spring-loaded, as someone already mentioned.
Part 2 (Option 2) With either rubber or rigid skirt, there would have to be something mounted similarly as the fender in 1. to "push" the skirt out when the wheel is turned sharply. It would not have to be "solid" like the fender, but could be just a small, curved bar. The bar pushes the flexible skirt from the inside, Get it?