I would welcome a gradient shader as it would help smooth out some harsh color transitions that you some times get on some types of terrain that use of Fractal Breakup alone doesn't fix and would be vary useful on some types of rock, that for now are hard to achieve.
The Shader would need to have good range and falloff controls and allow for both black and white and color gradients that must allow changes to the colors used like the way you can in Photoshop as way of example, if a separate node is out of the question could future versions of Terragen (Beyond the first commercial release) have this functionality built into existing nodes that deal with color such as the Surface Layer Node, but there maybe reasons, technical, developmental or other wise why this is not possible but since TG2 is modular in design I would have throat such things could be possible.
Regards to you.
Cyber-Angel