1. Joss specifically chose Doug Petrie for this issue, so while the other issues were a bit flexible regarding writing duties, there was specific intent in choosing Petrie to write #25.
Scott Allie: With Doug, I believe Joss wanted him tackling the particular beat he had planned for #25.
Petrie writes episodes that are game-changers. Huge moments for the characters that literally change the entire playing field. These episodes are always big plot points where secrets are revealed that relate to the greater themes of the season.
Episodes written by Petrie
Revelations - The gang discovers Angel is back.
Bad Girls - Faith starts to go over to the dark side when she kills the Deputy Mayor.
Enemies - The first iconic showdown between Faith and Buffy where they both pull knives on each other.
The Initiative - Riley is revealed to be a commando and that Spike had been chipped, no longer able to hurt people (that was a HUGE change)
This Year's Girl - Faith wakes up and switches bodies with Buffy, beginning her redemptive arc after a long absence.
The Yoko Factor - The Scoobies drifting over the season is pushed to the breaking point.
No Place Like Home - Buffy learns that Dawn is the key.
Fool For Love - Spike teaches Buffy about her death wish, Buffy learns her mother is extremely sick.
The Weight of the World - Buffy's fear of killing her sister manifests and her refusal to let Dawn die leads to her own sacrifice in The Gift.
Flooded - The theme 'life is hard' is introduced for Buffy.
As You Were - Buffy ends her self-destructive relationship with Spike, begins to rise out of her depression.
Two To Go - Willow's rage escalates out of control, no longer just targeting Warren but seeking revenge on Andrew and Jonathan and leading to her turning on Buffy and her friends.
Bring on The Night - The Turokhan attacks and Buffy gets beat down, now forced to realize they must become "an army" to defeat the First.
Get it Done - Buffy castigates and alienates her friends for being powerless, then receives her horrifying vision of the Turokhan army within the Hellmouth.
End of Days - The secrets of the Scythe reveal the history of the Slayer, Buffy begins her showdown with Caleb after she's reunited with her friends and the Potentials.
No throwaway funny episodes on this list above. So Petrie writing Issue 25 Living Doll means it's time to get serious, something big is about to happen.
2. Issue 20's Slay the Critics reveals that "Doug Petrie will wrap up this arc with another Twilight turning point."
3. Issue 25 features Xander and Dawn. Xander and Dawn have shared the most one-on-one time with each other throughout Season 8. Most of their time together has revolved around him comforting her about being a Giant and also getting her to open up about what happened with Kenny. Their interactions have all revolved around Dawn's issues where Xander is focusing on helping her (barring the battle scenarios of ToYL where after they talked, Dawn and Xander get all fight-y with the bad guys).
So how do Xander, Dawn and Twilight relate in what will be a huge turning point episode written by Petrie, an episode where secrets are revealed and the rules of the game change? It's all about magic. Dawn's entire life is being messed with by magic. She feels "like a freak" because she's no longer herself. So would it be fair to say that Dawn isn't a huge fan of magic at the moment? I think so. And who wants to end the age of magic? Yep, that nice super-powerful man, Twilight.
The title Living Doll could be read as very significant here. A synonym for "doll" is "puppet". So who is the "living puppet" in this scenario? I think it's Dawn and Twilight is going to manipulate her because of how magic has been affecting her. Dawn is the "living puppet" that magic has been playing with all season. Twilight might even be responsible for how magic has been transforming her. Either by sending Kenny to get close to Dawn or by watching Dawn from a distance (the way he's been spying on Buffy) and then putting the Giant/Centaur whammy on her.
The connection between Twilight and Dawn has been theorized and speculated on for a long time now, especially after the panel during #16 when Centaur!Dawn rides off into the setting sun (twilight). Looks like the connection between these two characters will finally be revealed in #25 Living Doll.
What does this potentially mean for Xander? Is his life in danger? Is he somehow connected to Twilight also?