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.
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.
There were a few novels that could be recommended, including Those Years Living in the Future World of Dragon Ball, If Dragon Ball, Latiz took Gohan away, and The Strongest Sun Gohan of Dragon Ball. These novels all used Gohan as the protagonist, telling his growth and adventures in the Dragon Ball world. These novels might satisfy your need for Gohan to be the protagonist.
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.