Instead of using only blackberries, try mixing in some raspberries and blueberries as well. Or replace the blackberries with sliced peaches or nectarines and a handful of dried sour cherries or cranberries. A combination of 3 or 4 plum varieties, all pitted and quartered, is also wonderful. If you like, add a pinch of freshly grated nutmeg or a dash of vanilla or almond extract to the fruit. Or sprinkle a small handful of sliced almonds or chopped pecans over the topping before baking.