Yes, there are likely 'Bardock Kills Frieza' fanfics out there. The Dragon Ball universe has a huge fan base, and many fans like to create alternative storylines. You can try searching on fanfic websites like FanFiction.net or Archive of Our Own.
The plot could start with Bardock having a premonition of Frieza's future actions against the Saiyans even before the destruction of Planet Vegeta. He then secretly trains and discovers a new form or power that allows him to stand up to Frieza. When Frieza arrives to destroy the Saiyans, Bardock reveals his newfound strength and battles Frieza. After a long and intense fight, Bardock manages to defeat and kill Frieza, changing the course of the Dragon Ball universe. This could also lead to new alliances and power struggles among the remaining characters.
There are fanfictions where Vegeta has a spiritual awakening of some sort. He realizes his true potential and his connection to the Saiyan pride. This newfound power allows him to generate a massive energy field around Frieza. The energy field then compresses in on Frieza, crushing him from all sides until he is completely destroyed. This method not only shows Vegeta's power but also his growth on a more spiritual level.
Their relationship could grow deeper as they face the challenges of pregnancy together. Krillin might become more in tune with 18's needs, and 18 could see a new side of Krillin's caring nature.
Bulma could also be a common character paired with Krillin. Bulma is very intelligent and could potentially help Krillin figure out how to time travel better or she might be the one who creates the time - travel device in the first place. And she could use her resources to help Krillin in his time - traveling adventures.
Frieza might initially be shocked. He's used to being a feared and powerful being, and the idea of having a family is very foreign to him. So he could be in denial at first.
In the manga, Krillin doesn't stop training. His sense of duty and the need to protect his comrades push him to keep improving his skills and strength.
For Yamcha, you can focus on his growth from being a bit of a comical character to a more serious and respected fighter in the context of his relationship with Frieza. Maybe he learns from Frieza's cold and calculating nature how to better strategize in battles. For Frieza, his development could be about him learning to care for someone other than himself. He could start to see Yamcha as an equal, which is a huge shift from his usual superiority complex.
One possible fanfiction could focus on their daily life together. They might have a sweet morning routine, like making breakfast and chatting about their plans for the day. Krillin could be shown as a very caring partner, always looking out for Android 18's well - being. For example, he might bring her a flower he found during his training, just to make her smile.