To do better than they already have would require making the car more like an Aptera. Do you think the market for fully-fendered three wheelers with little to no back seat is big enough for Tesla?
There's this myth that a "good designer" can make a car with any basic shape have a great drag coefficient. It doesn't work that way. The rear taper is one of the most critical aspects toward having a low drag coefficient. Aero drag consists of two components - skin friction drag and pressure drag. The latter is the area-weighted difference between the pressure at the front and the pressure at the rear (force = pressure times area). The larger the area of the low-pressure wake, and the deeper the low, the greater the pressure drag. It's unavoidable. You have to have a small rear cross section to have low drag because the force is proportional to cross-sectional area. You can increase the wake pressure with vortex generation, but everyone does this already. The only additional thing you have control over is the area, and that comes down to your taper. And you can't taper too fast or you get flow detachment.
The notion that a "good designer" can make a car with any basic shape be super-streamlined if they're clever enough is equivalent to the notion that a "good designer" can make airplane wings have any basic shape so long as they're clever enough. Brick-shaped wings, scoop-shaped wings, you name it - just need a clever designer to tweak some minor details and then they'll work as good as airfoil-shaped wings, right?
That's just not reality.