The gacha system might also be used as a plot device to move the story from one setting to another. Maybe getting a certain item from the gacha unlocks a new location or dimension in the DxD fanfiction world. This allows the author to explore different areas of the fictional universe and introduce new challenges and opportunities for the characters, which keeps the plot fresh and engaging for the readers.
One interesting concept could be the unique characters that can be obtained through the gacha system. For example, getting rare or powerful devils or angels that are exclusive to the fanfiction's universe. It adds an element of surprise and excitement, much like in regular gacha games.
Rias Gremory remains a popular choice. She is a central figure in DxD, and when combined with the Fate system, her leadership and her own powers can be explored in new ways. For example, how she might strategize against opponents who use Fate - based powers or how she might incorporate such powers into her own devilish abilities. Plus, her romantic and friendly relationships are always interesting to see in the context of this fanfiction.
Well, in 'Fate System DxD Fanfiction', the idea of how different Servants from the Fate series interact with the Devils in DxD is really cool. For example, a heroic spirit like Archer might have some really interesting exchanges with Rias or other devil characters. They could share their combat experiences or knowledge about different forms of power, which would be a great source of character development and plot twists.
Another theme is the exploration of different relationships. This can include friendships between characters from different series, as well as romantic relationships. For example, a character from Fate Stay Night might form a bond with a character from High School DxD. The fanfictions often explore how these relationships develop in the combined universe.