Hi CW -
Overall it's a very nice piece. Some comments/ideas:
1. Get things moving faster. I felt like the theme could be over in 4 or 6 phrases instead of 8 (right before the strings come in). Basically I felt like "I get the idea of this variation, I'm getting bored now."
2. It was very refreshing when we got to the minor, and then to the triple meter. Good ideas for variations - again, I would just like them to come a little sooner. See how it goes cutting out one (or maybe 2) of your phrases. I would rather have a 4-phrase theme with a lot of creative variations than a very long theme.
3. You can vary the harmonic rhythm. It feels very plodding at times, because the chords always change every 2 beats. Could be interesting to have a variation that goes at double- or half-speed (harmonically anyway). Or even just delaying one of the chords every now and then - hold one out for 3 beats, the next only 1 beat, and so on.
4. Don't be afraid to get far away from the melody, harmony, or rhythm you have established. All you need for it to 'count' as a variation is for one element to stay (more or less) the same!