Well, they might be really shocked at the power levels of Goku and Broly. RWBY characters are used to their own kind of battles, and seeing the insane energy blasts and transformations in the Goku vs Broly fanfic could leave them in awe. They might start thinking about how they could incorporate some of those powerful moves into their own fighting styles.
Another aspect of Daki fan fiction could be the alternate universes. What if Daki was never a demon? How would her life be? These are the kinds of creative ideas that can be explored in fan fictions, and they often attract fans who want to see different interpretations of the character.
Some of the RWBY villains might be intrigued by the strategic elements in Red vs Blue fanfiction. In RWBY, they are used to battles with semblances and dust, but in Red vs Blue, the battles rely more on regular weapons and teamwork in a different way. They could analyze these new strategies and perhaps even consider applying some concepts to their own plans in RWBY, if possible.
One popular theme is redemption. Since Daki is a demon, some fan fictions might explore the idea of her finding a way to redeem herself. This could involve her making amends for the harm she has caused to humans.
Nezuko is initially 12 years old in the manga. As the story unfolds, her age doesn't change significantly in a traditional sense, but her character development is quite remarkable.