This pumpkin bread recipe will settle a bit as it cools but should still retain a domed top. It's not going to be as domed as a conventional bread with gluten, which creates structure in breads. If you've tried other recipes that have sunk or have a flat top it it most often then too much leavening agent was used. If you add to much the baked good will rise too quickly and ten fall before its done baking or soon after it starts cooling. Try reducing the amount of baking soda and/or baking powder.