If you don't exclude it, the DL website normally shows you four classes, although basic economy disappears for the return if you haven't selected it for the outbound. Otherwise, you can select a different class for the return or in a multicity booking. If you want more control, which usually means breaking the fare and paying more, you can call DL and ask for exactly the segments and classes you want, including long overnight connection (up to 24 hours) on international itineraries.