Well, it really depends on the author's take in the fanfiction. Some might make the Sharingan equal to the Rinnegan. They could give the Sharingan new and enhanced abilities that we haven't seen in the original Naruto series. However, generally speaking, the Rinnegan is seen as a more advanced and powerful dojutsu. It gives access to a whole new level of chakra manipulation and powers that the Sharingan doesn't usually have. But again, in the world of fanfiction, anything can happen.
The Rinnegan vs Sharingan in fanfiction is an interesting concept. The Rinnegan is associated with the Sage of Six Paths and has a more god - like quality to it. It can manipulate the six realms which is a huge deal. On the other hand, the Sharingan is more common among the Uchiha clan. In fanfictions, if a character has a Rinnegan, they are often portrayed as a major threat or a very important figure. The Sharingan, while still powerful, is sometimes just a stepping stone towards the development of the Rinnegan in some fan - made stories.
In fanfiction, the Rinnegan is often depicted as more powerful than the Sharingan in many ways. The Rinnegan has abilities like the Six Paths techniques which can be extremely overpowered. For example, it can allow the user to control gravity, summon large creatures, and even revive the dead. While the Sharingan has its own set of cool abilities like predicting movements and casting genjutsu, it seems limited in comparison when it comes to the grand scale of power shown by the Rinnegan in most fanfictions.