One possible plot twist could be that Sebastian's true motives for getting Clary pregnant are revealed to be part of a larger, darker scheme. Maybe he plans to use the baby for some sort of power - play within the Shadowhunter world.
A really unexpected plot twist could be that the baby isn't actually Sebastian's. There could be some sort of magic or deception at play that makes everyone believe it's his, but in reality, it's someone else's, which would add a whole new layer of drama and mystery to the fanfiction.
The best chapter pointer here is Chapter 57, "Do you still see me as your brother in your eyes?". It opens with Lu Xingchao's numerous fans were stunned. and closes on I'm saying, big bro, can you at least reply to me?.
A practical answer is Chapter 46, because it turns the idea into a concrete scene. The chapter opens with Upon hearing this, everyone but Jiang Li perked up with high alert., adds pressure through There must be some mistake with the crew's calculation, I demand a..., and ends on Did you calculate.
Chapter 45 is the strongest chapter-level answer because it starts from Helplessly, Jiang Yan had no choice but to step on the accelerator. and follows through on The guests have to go through so many troubles on the way, while these....
The best chapter pointer here is Chapter 23, "Became Trending for Being Scolded". It opens with Although Jiang Li exerted very little strength, Jiang Yan instantly felt... and closes on However, just as she buried herself in the study, pouring over mysticism....
The best chapter pointer here is Chapter 25, "He's Actually Mr. Shen's Son?!". It opens with Therefore, from his childhood, Jiang Shixu had felt an extreme aversion to... and closes on Having worked beside Shen Lan for over a decade, she had been there when....
Chapter 50 is the strongest chapter-level answer because it starts from [666, if good scores determine rankings, is Flash Change's production team... and follows through on What kind of heavenly being had such a sour temper and liked dancing....
Chapter 61 is the strongest chapter-level answer because it starts from Hahahaha, oh my god, Jiang Li's mouth is even more venomous. and follows through on A cold shower is pretty normal, right?.
A practical answer is Chapter 65, because it turns the idea into a concrete scene. The chapter opens with Several minutes later, the room remained silent., adds pressure through But now this luxurious vibe was disrupted by the incongruous outfit into..., and ends on What's the deal with these.
Chapter 17 is the strongest chapter-level answer because it starts from Jiang Li glanced at Jiang Shixu, who was tense and anxious behind her, and... and follows through on I've been a mother for more than ten years but never tasted anything made....