Like many other adaptations of Young Adult novel series (Harry Potter, Hunger Games, Twilight), the plan was to split the last book in the series ('Allegiant') into two films, with this movie basically covering the first half, and Allegiant: Part II (2017) covering the rest. However, due to negative reviews and a disappointing box office result, it was decided to turn Ascendant into a TV movie instead, to be followed by a spin-off TV series. Shailene Woodley, Theo James and Miles Teller subsequently pulled out of the project.
Kate Winslet was so keen to return in any capacity as either a "flashback or sim[ulation]" that she got her agents to get in contact with producers to specify her availability. However, it was the producers who turned down the opportunity for the sake of time and storyline constraints.
One of the dream sequences Marcus Eaton has involves raining ash as a reference to the Holocaust film Schindler's List (1993)), which is coincidental as the grandparents of Veronica Roth's, the author of the Divergent books, were actual Holocaust survivors.
Tris and Four have a scene by gate "N20" in one of the terminals at O'Hare airport. This is fictitious, as the gates at O'Hare only go up to M, in Terminal 5. The only way to have N gates would be to extend the terminal onto the south taxiways and runway.