It could also change the power dynamics. Percy had unique powers and abilities that were crucial in the war. Without him, the opposing forces might gain an upper hand. For example, his control over water was a major asset in battles against the giants. His death would mean the loss of that strategic advantage.
One possible reason could be that she sacrifices herself to save someone else. Maybe she jumps in front of a fatal blow meant for Percy or one of the other important demigods. Another reason could be that she makes a miscalculation in her battle strategy. She might underestimate the strength or the trickery of the giants and end up in a situation where she can't escape.
In the fanfiction, Percy might face new challenges related to his demigod status. Maybe he has to deal with the aftermath of the war, like rebuilding Camp Half - Blood or helping new demigods adjust.
One possible plot could be Percy and his friends dealing with the aftermath of the war in the mortal world. Maybe they have to hide their demigod identities even more carefully as the war has drawn some unwanted attention. They could also be working on rebuilding Camp Half - Blood and trying to make it safer for new demigods.
Well, it would shatter them. Grover, who has been friends with Annabeth for a long time, would be heartbroken. He might even blame himself for not being able to protect her. The other campers at Camp Half - Blood would also feel a great sense of loss. Annabeth was like a guiding light for many of them, especially the younger demigods. Her death could lead to a drop in morale among the whole demigod community, making it harder for them to fight against the giants.
Perhaps some of the demigods from other camps that he didn't interact much with before could become his new allies. They might see his leadership during the war and want to be part of his group, to learn from him and fight alongside him in any future conflicts.
In fanfiction, Percy might finally get some peace. He could go back to school and try to have a normal life, but still be on guard for any new threats. Maybe he'd focus more on his relationship with Annabeth and they'd plan for their future together.
It could be a rogue demigod who was on the losing side of the Giant War and is now seeking revenge. This demigod might have found some dark power or be allied with some minor but dangerous mythical creatures.
Yes, in some fanfiction, Percy Jackson can be depicted as a Frost Giant. Fanfiction allows for a lot of creative interpretations, and authors might take the existing Percy Jackson universe and twist it in unexpected ways, like making him a Frost Giant.
There could be new demigods. Maybe some who were hidden during the giant war and now come out. For example, a demigod of a less - known god or goddess.