I am thinking of writing a Theme and Variations based on a sonata. With sonatas, I tend to gravitate towards the first movement, which is most often in sonata form, and this poses a bit of a problem. Namely, where to cut off the theme and start variations? With rondos and symphonies, this cutoff point is clear, but for different reasons. In a rondo, it is clear because it is the section that comes back multiple times. In a symphony, it is clear because of orchestrational changes that coincide with the change in theme.
With piano sonatas, not really, especially Mozart sonatas. Obviously, I would be including the first theme, but often the first theme ends on a half cadence, and the transition ends on the same chord as that half cadence, but is modulating in the process. This is what gives the smooth movement from one theme to the next in a sonata. So the first theme on its own or even combined with the transition feels like an incomplete entity, like it has been amputated, even at the last phrase ending. But I don't want to include the entire sonata exposition if I don't have to. Especially because the second theme tends to be more developmental than the first theme anyways.
As an example, take Mozart's Piano Sonata in F major K 280. These are the 2 possible cutoff points that don't include the more developmental second theme:
Either way, I'm going to have to add an F major chord at the end of the theme. I don't feel comfortable ending a melody at a half cadence or a modulation to the dominant, just to repeat the entire melody with embellishment, because then it feels like every variation is incomplete. But where should I cut off the theme, add the F major chord, and start the variations? At the end of the first theme and before the transition? Or at the end of the transition before the second theme arrives?
On the one hand, if I cut it off at the end of the first theme, I don't have to worry about embellishing triplets or having it feel too developmental(like mid theme developmental, not variation developmental). On the other hand, if I cut it off at the transition, I have more wiggle room for variations and thus can probably do more variations than if I just have the first theme alone. Mid theme development is also one of many reasons I don't usually use the slow movement as my source of melodic and harmonic material in a Theme and Variations.
So, once I figure out what sonata I want to do a Theme and Variations on, where should I cut off the theme, add a tonic chord, and start writing the variations? Should I cut it off at the first theme to avoid too much mid theme development? Should I cut it off at the transition to have more wiggle room for variations? Or should I let the length of the first movement of the sonata dictate that for me(long exposition, just the first theme, short exposition, first theme and transition)?