That's an interesting idea.  I just switched from an Android phone to an iPhone a few months ago, and I see that Apple does a similar thing for their photo controls:  You can click Edit-Light and have a quick slider, or click Edit- Light-DownArrow which gives you a menu of Brilliance, Exposure, Highlights, etc. many of which are manipulated in concert by the main Edit-Light slider (and there's a separate button for Vivid, Vivid Coll, Vivid Warm, Dramatic, Dramatic Cool, Dramatic Warm, etc., each of which combines adjustments from both the Light and Color menus). 

More complexity to program, but allows people to make simpler changes without having to experiment and get into all the complexity themselves.

Exactly. In the case of Pianoteq, I would see the small number of "global sliders" as a first line of editing. Further, finer tweaking could still be subsequently applied with the individual parameters. There could be a graphic representation (like a color overlay) showing which parameters are affected by a particular global slider. You would see them all move in real time as you changed a global slider.

I think those regular controls that are already there ARE the macro controls, in fact

Well, then, they are not macro enough!
"Supermacros"  I want...

More complexity to program, but allows people to make simpler changes without having to experiment and get into all the complexity themselves.

Here is my suggestion: why not, in a future version, implement a SMALL number of "global" controls that would simultaneously move multiple parameters to achieve overall changes which could be labeled (for example) "brightness", "expression", "resonance", "warmth", etc. These are evidently quite subjective, but at least they invoke something that most players can understand. In fact, the name of these global parameters don't matter so much because users would rapidly learn how they affect the sound, and how to use them to get closer to the sound they are looking for.

Let's take a simple example and call it "Brightness". The affected parameters could include: all three "hammer hardness", soundboard cutoff, equalizer, Effects section EQ, reverb tone, keyboard scaling, etc. A proper "cocktail" of all (or just some) of these could be tweaked by a single global slider. In the case of "expression", the global slider could simultaneously change dynamics, Compressor settings and the velocity curve. These are merely examples and I am sure the developers would know much better what to do...

BTW, I am NOT suggesting that these Global controls would be  replacing the existing individual controls! They would simply act as a fast, direct way to sculpt the sound (for example in a live situation, or when playing on a different set of speakers)...or when the mood calls for it!.  Like I said, they should come in a small number (something like five or so) plus a simple reset button to re-center them. Just a thought. Probably other people have had the same idea?