1. Britney almost played Allie in the The Notebook.
2. Paul Walker was supposed to be in the "Sometimes" video.
3. The original version of "Me Against the Music" did not feature Madonna, it featured Penelope Magnet.
You can hear her rap in this version. It's pretty good. You might remember it from when Britney performed on the NFL kickoff.
6. "Umbrella" was written for Britney.
7. There is an unreleased album called Original Doll.
8. Britney could have played Dawn in I Heart Huckabees.
9. There is reportedly a music video for "When Your Eyes Say It."
10. There was supposed to be a music video for "Hot As Ice."
11. Melissa Joan Hart could have been one of Britney's friends in Crossroads.
12. At the infamous 2007 "Gimme More" VMAs, Britney was supposed to sing a song called "Cry."
13. Britney was supposed to play the angel mom in Little Nicky.
14. Daft Punk was supposed to work with Britney in 2002.
In an interview with The Hollywood Reporter, Britney talked about meetings with Daft Punk. She said, "Mainly, I've just been having meetings with people to hear their stuff to see if I want to work with them in January. I met up with some really hot producers, William Orbit and Daft Punk, overseas."
15. Everything with "Outrageous."
16. Britney recorded songs with Fred Durst. That means Britney/Fred Durst songs exist.
17. The original concept for the "Baby One More Time" video was supposed to be an "animated Power Ranger-y thing."
18. The Oops!...I Did It Again album was originally supposed to be called Sunflower.
19. Britney almost started a production company and signed JoJo.
According to JoJo, "I went backstage at a concert she was doing and her lawyer called me up and offered me a production deal with a company that she was starting."
20. "I've Got That Boom Boom" was supposed to be the second single from In the Zone.
21. This "Break the Ice" remix featuring Fabolous was supposed to be the second single off of Blackout.
22. Britney shot a Proactiv infomercial that never aired.
It was reported in 2005.
23. Pink, Natalie Maines, Faith Hill, and Jennifer Lopez were originally supposed to be in the 2004 Madonna VMA performance.
24. The original concept for the Femme Fatale album was songs like "Unusual You."
25. Supposedly, the original concept of "Born to Make You Happy" involved a secret pregnancy.
26. In "Everytime," Britney was supposed to commit suicide.
27. The real "Gimme More" video.
H/t to this website for compiling a bunch of these. You should also take them with a grain of salt. Britney is a very mysterious entity.