The main character would surely be the 10th Doctor himself. His companions, depending on the era, like Rose Tyler or Martha Jones, could also be main characters. They play important roles in how the Doctor adjusts to his new form.
Obviously, the 10th Doctor is the central figure. But there could also be some recurring villains who show up to test the newly regenerated Doctor. And there might be some new characters created specifically for the fanfic, perhaps a character who helps the Doctor understand his new regeneration better or a being from a previously unexplored planet that the Doctor meets during this early phase.
The 10th Doctor is the main focus, no doubt. In addition to his companions, there could be other Time Lords, either from the Doctor's past memories or new ones introduced in the fanfic. There might also be some alien races that have a special connection to the Doctor's regeneration, like a race that has knowledge about the process and either tries to help or hinder the Doctor during this crucial time.
Chapter 768, "Sorry Everyone", is the clearest early answer because it turns Shen Lan's situation into a concrete plot event. The setup is With a "snap", Jiang Zhao Yuan's purple clay teapot shattered on the..., but the pressure keeps building through His face turned ashen, and he made to leave..
A practical answer is Chapter 4, because it turns the idea into a concrete scene. The chapter opens with Qiao regretted her words as soon as they left her mouth., adds pressure through It's difficult for us when you ask for a guest's room number and key right..., and ends on A little girl acting.
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,....
Chapter 830, "Pinky Promise for 100 Years", is the clearest early answer because it turns Jiang Bufan's situation into a concrete plot event. Early on, At eight o'clock in the evening, Jiang Li arrived home.. By the end, Immediately, two shadows darted out from the room inside, each....
Chapter 745, "I'm Pregnant", is the clearest early answer because it turns Sister Yan's situation into a concrete plot event. The chapter gives readers a real answer by opening on Several people finally met Shu Yan in the hospital ward.
A practical answer is Chapter 21, because it turns the idea into a concrete scene. The chapter opens with Jiang Li's clear and logical explanation had everyone stunned in place., adds pressure through Yinwan, don't worry, I won't let you suffer in vain., and ends on But the implications could be.
Chapter 857, "Interested in joining us?", is the clearest early answer because it turns Fuhu Village's situation into a concrete plot event.
Chapter 884, "Master?!", is the clearest early answer because it turns Wen Jiaojiao's situation into a concrete plot event. The chapter turns that into something concrete by moving from I've heard this temple is quite magical, but why are there so many...
Chapter 761, "Can You Take Us In?", is the clearest early answer because it turns Du Jiajia's situation into a concrete plot event. Early on, Lili, this is your maternal uncle's family..." Jiang Li lifted her.... By the end, Aunt Feng's expression changed, her voice lowered, "That must cost....
Chapter 701, "Let Bai Lu and Chen'an Terminate the Contract", is the clearest early answer because it turns Wu Daoshan's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Miss Jiang, that move of yours was totally awesome!" Shao Changqing...