You could use almost any combination of summer fruits in this, but sweet-tart ripe plums are perfect with tart raspberries. Choose red or green plums; prune plums come in to season later in the summer and work beautifully too. Peaches or apricots would stand in well for the plums, and earlier in the season, you could sneak in some rhubarb instead of the raspberries. Baking the topping for a few minutes while the fruit begins to cook makes it much more crisp.