The character of Tracy was eventually revealed as the "Mother" on How I Met Your Mother, but that role almost went to Ted's on-again, off-again girlfriend, Victoria.
Before the real Mother played by Cristin Milioti was introduced, Victoria was a long-term candidate in the eyes of many fans. The character, played by Ashley Williams, appeared in the first season of How I Met Your Mother as the baker that Ted met at a friend's wedding. They began dating and continued their relationship after Victoria took a cooking fellowship in Germany. The pair ultimately broke up - thanks to Ted's feelings for Robin - but she resurfaced in season 7 when the pair rekindled their relationship. Ted's feelings for Robin continued to be an issue with Victoria so they decided to break up for good.
Tracy was revealed as the "Girl with the Yellow Umbrella" in the How I Met Your Mother season 8 finale. She became a series regular for the show's ninth and final season, bridging some of the gaps that Ted hinted at in previous episodes. Although the response to Milioti was mostly positive, a final twist in the controversial series finale revealing that Tracy had passed away by the time Ted was telling his kids the long-winded story, and that it was all part of him realizing he's always been in love with "Aunt Robin," divided fans. However, that wasn't always the planned endgame.
Before How I Met Your Mother's actual ending played out, the show's creators had toyed with the idea to make Victoria the mother of Ted's two kids! The original show's order led up only to Victoria's introductory episode, suggesting that at one point in development, she was intended to be at the very least a stand-in for the Mother. Later, though, creators Craig Thomas and Carter Bays stated outright that had the show been canceled before season 8, Victoria would have been the Mother instead of Tracy. It was a contingency plan, with the writers feeling that they left open enough history between Victoria and Ted that it would make sense for them to end up together.
Of course, if Victoria had been the Mother, it certainly would have changed the perception of the entire series. Many of the details surrounding the Mother that Ted shared with his kids wouldn't have made sense had she been Victoria; her physical appearance, hobbies, or even demeanor doesn't match with what was used to describe the Mother throughout the series. On the other hand, going with Victoria - and keeping the Mother alive might - have satisfied viewers.
Beyond making the mystery fit, it would have been interesting to see what would have happened with Robin had Victoria been revealed as the Mother. If the creators went with Victoria but still kept Robin as Ted's soulmate, there would have definitely been some kind of conflict. On the other hand, if he and Victoria ended up together, Ted's friendship with Robin would have been nonexistent; Victoria made it very clear that she didn't want Robin in his life.
Maybe in some kind of alternate universe, Ted did end up with Victoria which could help select How I Met Your Mother fans as they can now focus on what could have been rather than what actually happened.