One popular Dark Gohan fanfiction might be those that explore his power in a new and extreme way. For example, stories where he battles against even stronger foes than in the original series. In some fanfictions, he might team up with unexpected characters while still maintaining his dark and powerful aura.
One popular theme is the exploration of Gohan's dark side in relation to Videl. For example, stories might focus on how Videl tries to bring Gohan back from his dark state. Another theme could be their relationship evolving in the face of Gohan's inner turmoil. Some fanfictions also play with the idea of them teaming up against new threats while Gohan is in his dark form.
Another aspect of his development in fanfiction is his relationship with his family. In some stories, he might reconcile with his father, Goku, or his mother, Chi - Chi. This can be a major turning point in his character arc, as it can bring out the more human and emotional side of Dark Gohan that was perhaps overshadowed by his darkness before.
It could be that a new threat emerges. Maybe another android, stronger than Cell, was observing the fight and decides to take advantage of the chaos after Gohan's death. This new android could become the main antagonist as it poses a greater danger to the remaining fighters.
One memorable scene could be when Videl first encounters Dark Gohan. The shock and fear on her face as she realizes this is not the Gohan she knows. Another is when she decides to fight beside him instead of against him, showing her loyalty and love. Also, a scene where Gohan, in his dark state, has a moment of clarity because of Videl's presence.
One possibility in the 'Gohan Trains After Cell Games' fanfiction is that Gohan is training not just for physical strength but also for mental strength. He has seen the horrors of the Cell Games and realizes that a strong mind is as important as a strong body. So he might be doing meditation - like training, learning to control his emotions better in battle. He could also be training to sense ki more effectively, which would give him an edge in combat against stronger opponents in the future.
One common theme could be the struggle between Gohan's good nature and the newfound dark side. Maybe it involves his relationships being strained as his dark side emerges. For example, his relationship with his family and friends might change as he becomes more aggressive or unpredictable.
I'm not sure exactly as I haven't read it. But it might be about a 'dark angel' and what happens in a post - dark world. It could involve themes like good and evil, survival, or redemption.
I'm sorry, I'm not aware of the author of this novel. There are so many novels out there that it's difficult to know every single one. You could search for it on literary websites or ask in book - related forums.