I'm not sure if there is an actual fanfic with that exact plot. Fanfic topics can vary widely, but this seems like a rather unfaithful and out - of - character concept considering Chi Chi's strong bond with Goku in the original series.
There might be some fan - created works out there that explore strange or alternate scenarios, but this particular 'Chi Chi cheats on Goku' idea goes against the general perception of their relationship. In the Dragon Ball universe, Chi Chi is usually portrayed as a loyal wife. However, the world of fanfic is large and diverse, so it's possible that someone, somewhere has written something along those lines, but it's not a common or well - known type of fanfic.
One reason could be to explore a completely different side of the characters. Maybe the writer wants to see how the Dragon Ball universe would react to such a scandal. Another reason might be for shock value, to get readers' attention with a very unexpected plot twist.
One idea could be that Goku forgets their anniversary. Chi Chi is initially angry but then decides to use this one - week period to remind Goku of their relationship. She sets up various challenges and tasks for Goku related to their past memories together. As Goku goes through these, he rediscovers his love for Chi Chi in a deeper way.
Power struggles can also be a common theme. Dark Goku is often depicted as very powerful. Chi - Chi, on the other hand, has her own strength in terms of her determination and will. The power dynamic between them can create interesting scenarios in the fanfic. There could be moments where Chi - Chi has to find ways to stand up to Dark Goku, or Dark Goku might try to use his power to manipulate or control her in some way.
Love and family are very common themes. Since they are a married couple and have a son, many fanfics explore how they navigate their relationship while also raising Gohan. For example, in some fanfics, Goku tries to be a better husband and father by spending more time at home.
Maybe in this fanfiction, Goku realizes that he and Chi - Chi have very different personalities. Chi - Chi is often more concerned with family and a stable life, while Goku is a free - spirited adventurer. This clash of values could lead him to decide not to marry her. For example, Chi - Chi may want Goku to stay at home and help raise Gohan, but Goku is eager to explore new planets and fight powerful enemies.
Well, in a one - week fanfic about Chi Chi and Goku, they might go on a training adventure together. Chi Chi could be trying to keep up with Goku's intense training regime, and there could be some cute and funny moments as she struggles. Maybe they also take some time to just relax and have a family picnic with Gohan. It would be a story full of action, family love, and the unique dynamic between these two Dragon Ball characters.
Another option is to look for fanfic review blogs or YouTube channels dedicated to Dragon Ball fanfictions. They might have reviewed some 'chi chi dark goku fanfic' and can give you an idea of which ones are worth reading. Keep in mind that the quality can vary greatly in fanfictions, so it might take some time to find ones that you really like.
One popular 'Goku x Chi Chi' fanfic is 'A Saiyan's Love'. It delves deep into their relationship, showing how Goku's simplemindedness sometimes clashes with Chi Chi's more domestic desires. It has great character development and some really sweet moments between them.
One reason could be his pursuit of stronger opponents. Goku is always eager to test his strength and find new challenges. In fanfiction, this might lead him to leave Chi - Chi to travel the universe in search of the toughest foes. He might feel that staying at home with his family restricts his growth as a fighter.
One possible plot could be that Bulma might be jealous of the relationship Goku and Chi - Chi have. Maybe she sees how much Goku has changed since being with Chi - Chi and she misses the old, carefree Goku. So, she tries to create misunderstandings between them by, for example, making it seem like Goku is more interested in training with her than spending time with Chi - Chi.