I'm totally happy with MC and I only could repeat my opinion after using Logic, Magix, Steinberg: Mixcraft is the best of them in usability and performance
Here are my suggestions that would be useful for common use:
1. Give us more built-in MIDI signal transformation possibilities (for example: velocity min/max/transformation curves etc, same for aftertouch and modulation wheels) to bring different controllers close to specific used VST Instruments on a track in playability. I'm using VeloScaler at the moment on each track, but this feels more or less like a workaround for doing this. This could be a unique selling point for mixcraft when a hammond or piano VST is also playable like the real one on each controller.
2. Make the UI only a bit more modern as other users says. But please do not lose the good MC usability. Some simple modifikation ideas with great effects could be: a) remove the gradients or glass effects to make it more flat designed, b) remove the really old school wood bars from the mixer and simply replace it with a carbon pattern or something like this, c) make the dialog for choosing Instruments a bit more attractive in font sizes and colors. This dialog is high frequently used and looks currently a bit like a linux commandline editor.
3. Better sounding VST effects would be nice (reverb, delay etc.) and a bit more interessting UI for this built-in effects.