The reveal is finally upon us!
Now, there are a few things to go in depth in this whole section, but they are better left for next episode, so for now, let's focus on trivia and smaller details!
-First off, Tsukihi's clothes. If you don't remember, that was the exact clothing set Karen was wearing when trying to look cute in the infamous episode 8 of Nisemonogatari, the toothbrush hell. She was wearing Tsukihi's clothes without permision and they looked too small and tight on her, due to her size.
-Also during Segoku's bedroom sequence, Nedeko mentions Araragi as "Koyomi-san" instead of "Koyomi-oniichan" for the first time in the series. This is seen as a confirmation of how she is finally trying to grow out of it. Tsukihi actually notices that honorific change, as you can see on the close-up they did on her ahoge, but decides not to pry on it.
Another thing on the same line of thought is how Sengoku now refers to herself as "me" and "I" and not as the childish "Nadeko" anymore.
We know how hair represents a lot in Monogatari, especially in Nadeko's case, who used her bangs and long hair to hide her face so others couldn't see her true feelings in her eyes. Her new hair style tells us how now she is more open to others, and how she tries to stay more true to herself.
-And out of curiosity, I don't remember if anyone mentioned during Koimonogatari, but Nadeko's manga was actually drawn in real life. The name is Kimi to Nadekko! and it has like, 3 chapters or so written by Nisioisin with art done by a famous shoujo mangaka. It's very cheesy and satirical of the shoujo manga industry, but it's a quick and fun read.