One common theme is adventure. Since Gohan has his Beast form, they might go on adventures together fighting enemies. Another theme could be romance, exploring their relationship deeper.
Most people would probably be very critical. The Beauty and the Beast is a well - loved story with certain values, and this kind of fanfic would seem to violate those values. It could also face a lot of pushback from the community for being inappropriate and not in line with the spirit of the original work. Moreover, it might be considered offensive to many who view the characters in a particular way.
Well, another place could be some dedicated Dragon Ball fan forums. There are often sections where fans share their own fanfictions or recommend good ones. Some popular Dragon Ball forums might have threads specifically about Videl and Gohan Beast's relationship in fanfiction form. You could also ask around in those forums if you can't find what you're looking for right away.
In this fanfiction, Gohan might learn valuable life lessons from Grandpa Gohan. Maybe Grandpa Gohan would teach Gohan some old - fashioned martial arts techniques that have been forgotten over time.
In such a fanfiction, perhaps future Gohan could warn present - day Gohan about upcoming threats that he has faced in his own timeline. He might share his experiences and knowledge, which could make present Gohan a stronger fighter and a wiser person.