Seven of Nine was supposed to be in a Star Trek: The Next Generation movie.

Seven of Nine was supposed to appear in Star Trek Nemesis and possibly further Star Trek: The Next Generation films, but actress Jeri Ryan turned down the role.

Seven of Nine was an extraordinarily popular character and after Star Trek: Voyager ended the producers of Star Trek Nemesis wanted to take advantage of that. Last night during the 50 Years of Star Trek special Jeri Ryan revealed that she had been offered a part in the Star Trek: The Next Generation film. Ryan says she turned down the role because she didn’t want to be pigeon holded as an actress

 Star Trek: Deep Space Nine’s Barry Jenner passes away at the age of 75.

If Seven of Nine had been in Star Trek Nemesis she would have replaced one of the characters in the film. I don’t think this means they would have written anyone in the cast out, but that Seven would have ended up replacing someone.
The obvious character for her to replace would of course be Data. Brent Spiner didn’t want to continue to play Data because he was getting older and thought it didn’t look right for him to be portraying the android. This was also the reasoning behind B4 replacing Data; B4 was of course originally supposed to be played by a different actor.
It’s obvious that the creators of Star Trek Nemesis wanted a plan in place to make further Star Trek: The Next Generation films without Data. And as much as I love Star Trek: The Next Generation, I’m really glad they didn’t go through with this plan. Star Trek: The Next Generation would be like Star Trek: The Original Series without Spock. It would still be most of the crew, but you’d be missing a very important piece of the puzzle. Explaining why Riker and Troi were back and not on the Titan would have been hard to do in future movies as well of course.
Jeri Ryan is fantastic, and I’m a fan of Seven of Nine, but it just wouldn’t have felt the same having Seven on the bridge in Data’s place.