Well, a really interesting plot twist might be if the Tenno from Warframe and the Guardians from Destiny had to team up against a common enemy that was a fusion of both universes' most dangerous foes. Say, a combination of the Sentients from Warframe and the Hive from Destiny. This would force them to learn about each other's powers and weaknesses very quickly. Also, perhaps a plot twist where the Light from Destiny and the Void from Warframe interact in an unexpected way, like the Void corrupting the Light or vice versa.
One possible plot twist could be the merging of Warframe's unique movement abilities, like bullet jumping, with Destiny's more traditional FPS movement. Imagine a Warframe character suddenly appearing in the Destiny universe and using their acrobatic skills to outmaneuver the enemies. Another could be the combination of Warframe's Orokin technology with Destiny's alien races. For example, the Vex could start using Orokin energy sources, leading to a whole new level of threat.
In a Warframe Destiny crossover fanfiction, a great plot twist could be the discovery that the two universes are actually two sides of the same coin. The Orokin might have had a connection to the Traveler in the past. Maybe the Orokin were trying to harness the power of the Light in a different way, and something went wrong, creating the Warframe universe as we know it. This would completely change the way both the Tenno and the Guardians view their own histories and their places in the cosmos.
A practical answer is Chapter 16, because it turns the idea into a concrete scene. The chapter opens with Just as the words fell, Jiang Shixu swiftly cast his eyes downward., adds pressure through Smelling the pleasant jasmine scent coming from her, his tense nerves..., and ends on Jiang Shixu.
If you want the clearest chapter reference, go to Chapter 17. Early on, Jiang Li glanced at Jiang Shixu, who was tense and anxious behind her, and...; later, the chapter leans on Okay, let's go eat." After speaking, she strode forward..
Chapter 776, "Sister Li Keeps a Toy Boy?", is the clearest early answer because it turns Weibo's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Apart from the two of them, some other users had started following...
Chapter 646, "This person deserves a slap in the face!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Yan: "....." Bloody green tea, get lost!
The best chapter pointer here is Chapter 1, "I'm Not Interested in Uncultured Men". It opens with Jiang Li, I'm warning you, don't set your sights on Yinwan anymore, or... and closes on Yet before she had the chance to teach her a lesson, the male lead arrived....
Chapter 909, "Hexin Sanatorium", is the clearest early answer because it turns Jiang Buchen's situation into a concrete plot event.
Chapter 18 is the strongest chapter-level answer because it starts from In the corner, a girl in a black dress sat quietly, holding a glass of... and follows through on Hurry home, what is a young lady like you doing running to a place like....
The best chapter pointer here is Chapter 10, "All for Wiping Your Butt". It opens with Qiao's shocked face struggle to recover, Lin Manru also tasted for the... and closes on Who was it again saying that the Jiang Family's tradition was lacking,....
If you want the clearest chapter reference, go to Chapter 23. Early on, Although Jiang Li exerted very little strength, Jiang Yan instantly felt...; later, the chapter leans on The pain twisted his face grotesquely as he dove headfirst into the sofa..