I have been thinking that it was rather odd that Bethesda trademarked the name a few months back before dawnguard was released, in earlier talks with Mr.Todd Howard he stated they were working on dlc and maybe he was talking about dawnguard but what if he was talking about the major dlc and maybe dawnguard is just the knights of the nine of skyrim wasnt even thought of at the point that Dragonborn was trademarked