Which franchise/series has done the best job of keeping a coherent storyline across multiple games?
If you were to ask me this question before the release of Ultima IX my answer would have been different, but since that franchise has devolved into a state of utter nonsense, I’d have to say that The Elder Scrolls series has done the best job of keeping a coherent storyline across multiple games.
The Elder Scrolls series is renowned among both critics and fans for its high attention to detail, attempted realism, and the vast number of names, dates, and places that constitute its history. Many aspects of its societies, cultures, and religions remain consistent over numerous entries because the games themselves have a tendency to individually focus on one piece of the greater pie. They all are set in the fantasy world of Nirn, on the continent of Tamriel, but only use one region within Tamriel as a launching point (The Elder Scrolls: Skyrim takes place in Skyrim, for example). Tamriel is divided into nine provinces (Cyrodiil, Morrowind, High Rock, Summerset Isles, Hammerfell, Black Marsh, Skyrim, Valenwood, and Elsweyr) and each region has its own native race (Imperials come from Cyrodiil, Dark Elves from Morrowind, Redguards from Hammerfell, etc.). There has been a ruling dynasty throughout the Third Era, but with the death of Martin Septim in Oblivion, a new Elven dominion was left to take over most of Tamriel, leading to separatist conflict seen in Skyrim. It is this kind of narrative consistency that tends to carry over game-to-game.
The Elder Scrolls themselves play a consistent yet somewhat limited role in each story, serving more or less as a framing device. Up until more recent entries, they had only been alluded to in-dialogue or through found journals and books, but Skyrim presents them as “fragments of creation” full of vast knowledge. This falls in line with how previous titles in the franchise have characterized them, as in Oblivious they were presented as remnants of the old gods (the Aedra – those that formed the world of Nirn).
Joinable organizations and guilds also remain consistent game-to-game despite the fact that some games are set hundreds of years apart. Hell, many of the racially and religiously driven conflicts still manage to occur after numerous centuries.
It’s this attention to detail and respect for the already established timeline that sets The Elder Scrolls apart from most other franchises. To that end, many series would be made better by creating a bible worth of events, conflicts, and figureheads to reference in future entries.