I think it would be pretty easy to improve the 600 RT.
First, add the option of using 4AA's or a more powerful long lasting rechargeable Lithium Ion battery.
Second, include an internally stored retractable, rotatable and extendable articulated arm to enable the flash head to be extended to a higher position, in relation to the lens axis, when the camera to which it is attached is held in the vertical orientation. Although this can be accomplished with external arm attachments, it would be nice to enable it with just the flash unit alone. It could also allow for other interesting positions in the horizontal orientation as well, such as one with a higher elevation above the axis of the lens, or one higher and more "to the side" as well..
Third, with the inevitable creep toward video uses in mirrorless cameras, it should also include a practical constant light source, a daylight/tungsten LED, as well as a conventional flash tube source.
Fourth, offer more and better light modifiers, maybe, as many have suggested, through magnetic attachment. Include a real external zooming fresnel modifier - not just the typical static fresnel that is typically built into the face of the lighthead, to make more precise light control possible.
Five, allow for a higher level of light output from the flash tube, at least a half stop more than with the 600 RT