Dark BrotherhoodMembers of the Dark Brotherhood seen under the city of Mournhold were last seen in Daggerfall when the Dark Brotherhood was a guild you could actually join. Their leader, the King of Worms, ascended to godhood and it is something of a mystery as to who is pulling their strings now.
GoblinsThe goblins found under the city of Mournhold were last seen *way* back in Arena. As a nice nod to their roots, the goblins in Tribunal even make the same frog-like noises as they fight with you as they did way back in Arena so many years ago.
ReferencesIn Morrowind, there were two (almost) naked Nords in the game, one who wanted you to chase a witch and one who was paralyzed. In Tribunal, there's a naked Nord in one of the gardens who firmly denies any connection with wizards or witches, and says it's just because the weather is quite hot around there. If the player pushes the topic, the Nord goes into a rant about stereotypes and how a naked Nord can't get any piece around these parts.
Information also contributed by Jason Musgrave