Every CFL (that's Compact Fluorescent Lamp) I've seen, aside from the ones that mount in a fixture that's driven by an external ballast, has its ballast built into a capsule that sits at the bottom of the coil of the fluorescent tube, or at one end of a doubled-back tube. This one-and-one construction
necessitates a form factor that was as recently as ten years ago much larger than a conventional lamp. The form factor comes closer to approximating the typical incandescent now, but the CFL is almost invariably larger than an incandescent of similar output. Also, the non-radiating nature of the control capsule guarantees a dark section in the lamp's coverage. While an incandescent obviously cannot shine where its electrical connections are, a CFL is actually casting its own shadow in some applications.
Speaking specifically to the coiled design, my idea is to place the electronic ballast within the confines of the fluorescent coil, thus minimizing the ballast's effect on the lamp's radiating area and enhancing the CFL's ability to fit into older fixtures that were designed with only the A21 lamp style in mind.
Some difficulties can arise in that the tube must be sealed, and installing it onto a ballast that is concentric within the coil's perimeter could be tricky. Most of the CFLs you see now have tubes that, once done wrapping around in a coil, are bent down to project straight along the bulb's axis into the ballast capsule. In my idea the coil is mounted separately on a cap and sealed there, the cap is then attached to and provides a connection with the electronics capsule, which brings with it the medium screw base. A simple click-on construction allows the two pieces to be permanently assembled and the unit is ready for installation.
For heat control inside the ballast capsule, its outer shell is wrapped with foil to reflect heat. It doesn't hurt any that this will also reflect most of the light landing on the capsule.
A couple of small changes to the design of the coil's attachment could permit easy replacement of the coil while retaining the ballast, which would be an environmentally repsonsible thing to do.