Effect options
For the synthesizer and piano sounds especially, something that should be considered in the next version of Musescore is a reverb, resonance, and cutoff feature that can be accessed from the "Create", "Notes", or "Edit" menus" then the selection would be named "Effects" or "Note Effects" depending on which menu the selection is located in. This would allow a cooler sound if tthe keyboard synthesizer is selected especially for previewing scores and placing notes. Also, maybe in the near future after a version or two, mixing one sound with another could be possible (e.g. synth with electric piano).
I am sure MuseScore would become more popular if these options/features were added.
Comments
None of these effects are present in the GM specification, so it is unlikely that a GM Soundfont would respond to them.
Reverb (and chorus) parameters are already accessible from the Synthesiser window.
Also bear in mind that MuseScore is currently primarily an application for displaying and printing music notation, and playback is very definitely playing second fiddle at the moment.
There is sequencing software available which can take a MuseScore generated MIDI file and manipulate it in the manner you describe, and also have far more control over it than MuseScore would give.
This doesn't mean that sometime in the future MuseScore will not offer enhanced playback facilities, but, given the amount of work still left to do on the proper display of music notation, it's likely to happen much later rather than sooner.