One common theme is the idea of identity. In Code Geass, Lelouch has to hide his identity as Zero while in High School DxD, the main character has to deal with his new identity as a devil or whatever supernatural being he becomes. In the fanfics, this is often explored further, like when a character with a Geass has to hide it while also dealing with their new identity in the DxD world. Also, loyalty is a theme. In both series, there are groups and factions, and in fanfics, characters have to decide where their loyalties lie when these two universes collide.
Well, a major theme is the clash of different worlds. Code Geass has its own political and power - based world, while High School DxD is more about the supernatural world of devils, angels, etc. In the fanfics, you see how these two different worlds collide. For example, the strict military and strategic world of Code Geass meeting the more chaotic and free - flowing world of High School DxD. And there's also the theme of morality. The actions that are considered moral or immoral in each series are put to the test when the two are combined in fanfics. For instance, using the Geass power in the DxD world might be seen in different ways depending on the different moral compasses of the characters.
A practical answer is Chapter 23, because it turns the idea into a concrete scene. The chapter opens with Although Jiang Li exerted very little strength, Jiang Yan instantly felt..., adds pressure through So the moment the news broke that Jiang Li was entangling with Shang..., and ends on However.
Chapter 924, "Competing to Auction Themselves", is the clearest early answer because it turns Jing University's situation into a concrete plot event. It opens with Not only did the netizens feel like they were on a roller coaster,...
Chapter 14 is the strongest chapter-level answer because it starts from Lin Manru had a rare good night's sleep. and follows through on But clearly, Lin Manru was unable to accept that her delicate daughter had....
The best chapter pointer here is Chapter 11, "She Smiled, He Panicked". It opens with After a meeting, Shang Shaojing pinched the bridge of his nose, tired, and... and closes on Miss Qin, please accompany us to the police station to explain this....
Chapter 771, "Just Friends", is the clearest early answer because it turns Xiao Xueyi's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Su Yinwan had lost five million followers in less than a week.
Chapter 751, "Even if both perish, he must pay the price!", is the clearest early answer because it turns Fang Wenzhe's situation into a concrete plot event. It opens with She cried out for heaven and earth, but her beloved was just...
If you want the clearest chapter reference, go to Chapter 24. Early on, Not only that, even the non-mainstream photos she had previously posted on...; later, the chapter leans on Phrases such as "ugly girl craving swan meat," "arrogant, wasteful....
Chapter 769, "Su Yinwan Gets into Trouble!", is the clearest early answer because it turns Xiao Xueyi's situation into a concrete plot event. Early on, The Jiang Family's banquet caused quite a buzz online.. By the end, Take a look yourself." Xiao Xueyi turned on her phone and handed it....
Chapter 767, "Jiang, Jiang Yu?!", is the clearest early answer because it turns Jiang Cheng's situation into a concrete plot event. Early on, Then, Zhao Lang felt a different kind of gaze and turned his head,.... By the end, Zhao Lang seemed to enjoy it too, chatting about business with the....
Chapter 749, "What if He Made a Fatal Mistake?", is the clearest early answer because it turns Shu Yan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with I saw the trending topics." Jiang Li left Weibo and initiated a voice...