Well, in a Danny Phantom ice core fanfiction, the ice core could be from a long - forgotten ghost realm. Danny might have to travel to this realm to understand the significance of the ice core and face challenges there. He could encounter new types of ghosts that are native to this ice - bound realm and have to use his wits and powers to get out alive. For example, these ghosts could be made of ice themselves and have the ability to freeze Danny's ghost powers temporarily. He would then have to find a way to thaw out his powers and defeat these foes.
A possible plot for Danny Phantom ice core fanfiction is that the ice core is a key to stopping a new villain who is planning to use the power of the ice to freeze the entire world. Danny, with the help of his friends Sam and Tucker, tries to figure out how to use the ice core to stop the villain. They might face difficulties like the ice core being too powerful for Danny to handle at first, or the villain trying to steal the ice core from them.