What I would find really useful would be a keyboard on which the currently depressed notes were displayed clearly. The programmer/designer on the Pianoteq one seems to have been aiming at some sort of quasi-realistic 3-d representation of what a piano keyboard looks like, which just seems pointless - cheesy even - to me.
<rant>What is it with the Pianoteq GUI design? It's as if the target audience was conceived to be a bunch of sad fantasists, salivating over the dream of being seated at the controls of a Model D - all these dreamy pictures of pianos, hammers and strings, and other silly bits of 'eye candy' - oh please! - something a bit/lot more grown-up would be a very welcome improvement. Or maybe the 'fantasy GUI' could just remain as a preference, for those who can't do without?</rant>
You can just about see which white notes are depressed but the black ones are very indistinct.
The perfect solution would be a re-sizeable keyboard - detachable if you like - on which you could customise to some extent the way in which depressed notes are (clearly) displayed.
I've tried the vmpk thing that varpa mentions but it's not really what I'm after. For one thing you can't easily see which black notes are depressed (again!) because of a half-hearted attempt at what I think the programmer took to be 'realism'. And it doesn't seem to respond to the damper pedal - unless that's some config issue which I didn't find. Also, the way in which it colours the whole of depressed white keys (blue) is a little distracting. Personally I'd rather just have something like a red dot on the key. That way you would still be able to see the pattern of white/black keys clearly at the same time as having the depressed keys clearly marked.
It's a diagram, not a pretty picture or animation, that I'm after.
N1X - PT Pro - Linux