What flavors complement pistachio (pudding specifically)?
I have used orange as an excellent flavor pairing with pistachio in cakes, biscottis, panna-cottas, custards, etc. I always add the zest of orange and a few drops of orange essence.

I've also tried cardamom and saffron with pistachios specially in Indian desserts and it is a very popular pairing in India. Just use a few strands of saffron and powdered cardamom seeds in a bit of warm milk before mixing in the rest of the ingredients for color, aroma and flavor.