Sequel Gap: The six-year gap between Daggerfall and Morrowind is the longest between games in the main series to date.
What Could Have Been: The blight was to originally be an expanding threat, and Dagoth Ur was originally to be joinable. Technical limitations axed the former, and time limitations axed the latter.
Word of Dante/Word of Saint Paul: Some of the game developers have posted "obscure texts" on the official forums regarding a number of in-game topics and characters. They are generally regarded by the fandom as canon. Michael Kirkbride, in particular, wrote a number of them which fill in gaps around The Tribunal, Vivec, and the Dwemer. (Kirkbride is also the author of many of the in-game books, including most-famously the 36 Lessons of Vivec series.)