As you know from my previous question about tempo conservation and time signatures, I am writing a Scherzo in Eb major. Now, if I were writing this for solo piano, I would gladly do an Eb to Ebm key change, since most key changes I see in Scherzi when going from the A section to the B section(or to put it another way, when going from the Scherzo to the Trio) are parallel key changes(i.e Cm to C or F to Fm).
But, I'm not writing this for solo piano, I'm only improvising on the piano. I'm actually writing the Scherzo for a string quartet ensemble. That makes me think that Eb minor is just going to make things difficult on the performers for no good reason. I mean seriously, how often do you hear string instruments in 6 flats? But, I feel that I still need a major-minor shift for contrast. Second minor key I could think of using would be C minor. Now that isn't so hard for string instruments and there is quite a bit of string quartet repertoire in C minor.
But you know, the relative key switch is used very often, especially from Beethoven onwards. Beethoven sure loves his Cm -> Eb -> Cm motion. Not saying that it's damming or anything to move to the relative key, but maybe in a Scherzo I would want something more interesting to add some humor to the piece. I know I'm already adding humor through rhythm and dynamic, but why not do it through the key as well? After all, the word Scherzo translates to joke.
There are 2 more possibilities I can think of, those being the mediant minor of G minor and a Circle of Fifths motion to some key like F minor(or maybe even a direct modulation to F minor). Somebody else suggested maybe moving to A minor, the minor key a tritone away from Eb, but I don't know that I would want to do that. It feels too distant to me.
For more context, I tonicize Bb several times in the Scherzo section and even fully modulate to Bb major. I'm also thinking of starting the middle section of the Trio in the key of Bb major to provide a sense of hope, a ray of light in the darkness of minor.
So now I'm back to what key to use. I don't think I want to use Eb minor because of how difficult it is. C minor is such an obvious choice. G minor, the mediant of Eb major, now that might be more interesting to use since the mediant is the least commonly used diatonic relation and because it sort of reminds you of the Bb major that previously occurred. On the other hand, since Bb major is the relative major of G minor, that movement to Bb major in the Trio, that ray of light, becomes much less interesting than it does if I use C minor instead.
A direct modulation to F minor might also be interesting, but is F minor going to fit with the Allegro Vivace? I find F minor to be a very lamenting, melancholic key. When I hear F minor being dramatic(like the stormy kind of dramatic, not deathly lamentation), I always hear quite an emphasis on its minor dominant, C minor and I wonder whether it is the tempo or that C minor relation causing the F minor to feel stormy.
So should I use the key of Eb minor since the majority of Scherzi have this parallel key switch when going to the Trio? Or should I use C minor because it is easy to play and has what I think is an interesting relation to Bb major? Or should I move to the Mediant minor of G minor because it kind of reminds you of the Bb major that has previously appeared in the Scherzo? Or should I directly modulate to a different related key such as F minor?