Sasuke might initially be shocked. He has always seen Sakura as someone connected to Konoha, so her leaving would be unexpected. He could be concerned about her safety, especially if she leaves without proper preparation or protection.
Sakura's reaction would depend on how the author writes her character development. If she's become more confident in herself, she might approach Sasuke with a more assertive attitude. She could tell him that she's not the same girl he left behind. However, she'd still have that underlying love for him. She might also be worried about how the village will accept Sasuke's return and how it will affect her standing in Konoha. So her reaction would be a complex blend of personal feelings and concerns for the village.
Naruto would be devastated. He has always considered Sakura as a dear friend and teammate. He would probably try to convince her to come back at first. If that fails, he might see it as his mission to bring her back by force if necessary, believing that she has been led astray. Sasuke, on the other hand, might have a more complex reaction. He has his own issues with Konoha, so he might understand Sakura's desire to leave on some level, but he would also be suspicious of her motives for joining Akatsuki.
Another reaction could be that she tries to find a way to redeem Konoha. Despite the betrayal, Sakura might still have some love for the village. She could work towards changing the situation from within, trying to fix what has been broken and make Konoha a place she can be proud to call home again.
She might be shocked at first. Sakura has always known Naruto as someone who is very attached to Konoha. If his leaving is because of her, she could feel guilty. She might try to stop him, asking him why he's doing this and telling him that he doesn't have to go.
Shikamaru, being the intelligent strategist, would analyze the situation carefully. He would consider all the possible moves Orochimaru could make and try to anticipate his next actions. He might also try to figure out Orochimaru's motives for taking Sakura hostage in the first place.
I'm not entirely sure specifically what 'sakura in founders time fanfiction' is about. It might be a story that combines the image of sakura, which is often associated with beauty and transience in Japanese culture, with the idea of founders, perhaps founders of a particular place or group. It could be a creative exploration of how sakura might have influenced or been part of the founders' story.
The founders might be quite surprised by the Golden Trio. They could be impressed with their skills and knowledge. For instance, Hermione's vast magical knowledge would surely catch their attention.
Sure, there are fanfictions where Sakura isn't from Konoha. Fanfiction gives creators the freedom to reimagine the Naruto universe. Maybe in a particular fanfic, Sakura hails from a small, isolated ninja village that has been at odds with Konoha in the past. This origin would color her view of Konoha and its ninja, and would also change how she is perceived by the Konoha characters. It's all about exploring new creative directions within the Naruto fanfiction world.
I don't really know who the main characters are in 'sakura in founders time fanfiction'. It could be that there are both fictional characters created specifically for this fanfiction and the founders who are the main focus. Maybe there are also characters who interact with the founders and the sakura in some way, like local villagers who have a connection to the sakura trees and also cross paths with the founders.