Combine sugar, zest, and cayenne (if using) in large bowl. Using rubber spatula, press mixture into side of bowl until sugar becomes damp, about 30 seconds. Gently toss melon, mango, and ginger with sugar mixture until combined. Let stand at room temperature, stirring occasionally, until fruit releases its juices, 15 to 30 minutes. Gently stir in raspberries. Stir in lime juice to taste and serve.

Serves 4