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.
Perhaps Gohan's own overconfidence could lead to his death. He might underestimate an enemy's power or a dangerous situation and make a wrong move that results in his end. For example, he could rush into a fight without proper preparation thinking he can handle it easily.
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.
Gohan's power of transformation could be affected. Instead of turning Super Saiyan in the normal way, it could be related to the haki system in One Piece. Maybe his Super Saiyan form gives him a special type of haki that is unique and extremely powerful.
Gohan may become more confident in relationships. In a harem situation, he has to deal with multiple people interested in him, which could boost his self - assurance.