I think it would be quite possible to keep the sense of history, and yet keep it simple enough that people can just tune in and understand what’s going on. Now, the example I’m going to give may have the advantage of working with a more well-known character than the Flash, but bear with me:
The Mask of Zorro.
In this movie, Antonio Banderas’ character becomes the new Zorro, taking over for Anthony Hopkins. By the end of the movie, Catherine Zeta-Jones is getting into the mask-and-sword act. Sure, people knew who Zorro was going into the film: a swordsman who wears a mask and fights injustice and oppression. The important details (his family and estates being taken from him, the vendetta against the man who took them, etc.) were (I believe) specific to this film, and were able to fit into a normal-length movie and still leave room for a plot.
So I can see a 2-hour pilot that sets up Barry Allen having been the Flash for however many years, and during the course of the story his nephew Wally West (in his late teens or early 20s), one of the few people to know his secret identity, is struck by lightning and gains similar powers. Barry starts training him, and by the end of the pilot he sacrifices his life to save the world (or at least some huge number of people) and Wally decides to carry on his mentor’s legacy.
If they wanted to, they could tie it into the previous series. It’s been about 10 years, they could get John Wesley Shipp to reprise his role as Barry for an episode, and state that Iris came back sometime after the end of the series. Or if they wanted to link Barry and Tina (there was some speculation), her original name could also have been West.
So it’s at least possible. Whether someone will be willing to try it remains to be seen.