The character i suspect will be in a DLC is Desmond Lockheart from Point Lookout.There's a few reasons Although he can be killed this doesn't matter you could have killed Tandi in Fallout 1 but the developers choose the canon so she lived in the canon fallout and became the President of the New California Republic in Fallout 2.The same points can go for any character in Fallout 3 it doesn't matter if you killed a character its up to the devs to decide if they lived or died.So back to my theory in Point Lookout Desmond was searching for Calvert his rival but more importantly Desmond wanted Calvert's advanced technology which he claimed calvert stole from him.So if i where to guess i say he will appear in the Old World Blues DLC which is set in the Big empty as is supposed to have outstanding technology Desmond being a tough son of b''''''''''ch , could well have made the journey to the Big empty in search of another rival this being Odysseus who was a made AI meant to appear in Van Buren who controlled the big empty but obsidian may have retconed Odysseus to be an AI with the brain patterns of a person desmond knew.So once desmond arrived at the big empty it locked him in and once the courier arrived it locks the courier in as well the courier then will have to help desmond and him/her self to escape while searching for clues about Ulysses who was at the big empty prior to Dead Money.