Maybe an ancient force, like a corrupted Isu remnant that has been awakened by the disruption in the Animus caused by Ezio's visions of Altair. This force could be trying to rewrite history to serve its own dark purposes and sees Ezio and Altair as the main obstacles in its path.
It could be a Templar who has also found a way to travel through time. This Templar might be trying to stop Ezio and Altair from uniting their knowledge and skills, fearing that they would become too powerful and expose the Templars' long - term plans.
Another possibility is a rogue Assassin. This Assassin has turned against the creed due to some past grievances. He sees Ezio and Altair's meeting as a threat to his own plans of creating a new order under his own rules. He tries to undermine their efforts and turn them against each other, while also trying to gain power for himself within the Assassin - Templar conflict.
A practical answer is Chapter 86, because it turns the idea into a concrete scene. The chapter opens with After over an hour of fuss, it was finally time to eat., adds pressure through Having tasted the tomato and eggs again, Wen Jiaojiao sighed in relief,..., and ends on But then her expression.
Look to Chapter 113 for the clearest answer. It begins with The next day, Jiang Lili was up before dawn as usual., keeps building through But this time was different; as she got dressed and opened her room door,..., and lands on If things are really as you say, that is indeed quite serious..
A practical answer is Chapter 88, because it turns the idea into a concrete scene. The chapter opens with She wanted to refuse, but didn't have the courage to do so., adds pressure through Jiang Yan immediately stepped forward, "I have one point, quick, quick,..., and ends on Let's go," Jiang Li.
Chapter 89 is the strongest chapter-level answer because it starts from Director Sun took a step back, "What's the rush? and follows through on If anyone should be mocking her, it should be me.] Wen Jiaojiao stood to....
A practical answer is Chapter 102, because it turns the idea into a concrete scene. The chapter opens with When the camera cut back, the three of them had already returned to the..., adds pressure through Sadly, his forehead was injured, wrapped tightly in bandages, so she..., and ends on The.
The best chapter pointer here is Chapter 124, "Officially Certified 250". It opens with Since there were too many points cards inside Jiang Li's barrel, Director... and closes on No need to repeat that number three times.] [Hang on, didn't Jiang Yan....
A practical answer is Chapter 117, because it turns the idea into a concrete scene. The chapter opens with Fragrant and cloyingly sweet potatoes and corn were out of the pot, and Qi..., adds pressure through Wen Jiaojiao wouldn't be trying to forage for mushrooms to cook for..., and ends on Since.
Look to Chapter 107 for the clearest answer. It begins with On the plane, when He Yun had taken off his mask and greeted her..., keeps building through She had also figured out his current predicament of being trapped by..., and lands on Although Su Yinwan's recent change in temperament had been.
If you want the clearest chapter reference, go to Chapter 89. Early on, Director Sun took a step back, "What's the rush?; later, the chapter leans on It's true about the go-kart costing one point, but this time we're using....
A practical answer is Chapter 89, because it turns the idea into a concrete scene. The chapter opens with Director Sun took a step back, "What's the rush?, adds pressure through The camera isn't on us right now, and Director Sun doesn't know about our..., and ends on If anyone should be mocking.