The initial Gotham City.
During the initial years of the movie production ‘continuity’ was not that important of a term to the movie-makers. The location, scenes and plots used to get varied from time to time and from movie to movie.
During the 80’s and 90’s era the appearance of the Gotham City changed every time a new movie was released. Later, it was the release of the X-men series when the writers understood the importance of ‘continuity’.
The directors and producers tend to ignore the ‘continuity call’ from the public, it was not till the release of X-Men: Days of Future Past, a whole new concept of interrelated timeline came into existence.
“X-Men: Days of Future Past attempted to course correct the timeline by introducing time travel, sending characters back in time to retcon the timeline in order to fill in a lot of plot holes.”
The major timeline of the X-men series started with the release of the film ‘X-Men Origins: Wolverine ‘ which formed the early reference source, being the earliest in the series.
“during that opening montage and the prologue to The Wolverine — baby Logan kills his dad, Sabretooth and Logan go on fighting every war ever and Logan saves Yashida from Nagasaki — the events of X-Men: First Class
The earliest of these, however, is Erik (future Magneto) in the concentration camp run by Sebastian Shaw, the future leader of the Hellfire Club. Shaw murders Erik’s mother in front of him.”
And that is exactly where the diversity in the timeline began. The original timeline depicted Bolivar Trask working on the first Sentinels.
“Mystique murders him in cold blood, which leads to his men capturing her and using her DNA to help create the Sentinel X models that would, half a century later, be used to overtake the planet
“.However, in the alternate timeline, Trask isn’t killed by Mystique which gives her a ‘messiah’ appearance. This leads to the formation of a new timeline, which formed the events in the movie- X-Men: Apocalypse, Dark Phoenix, the Deadpool.
The theory witnesses a lot of loopholes in the plot giving rise to the various fan-made theories. With the ending of the X-men franchise after the Dark Phoenix, we can expect the creators to work further on their timelines.