A good Sample and Hold (DivKid Rnd Step or an MI Kinks for example) and another voice (I'd pick a digital one) would let you add harmonies and layers, that's probably the direction I'd head in. In a rack this size I'd also be tempted by a Disting and by a Links to add some modulation mixing in a small packacge.
my tunes: https://stevehand.bandcamp.com/