Chiron, the centaur mentor at Camp Half - Blood, might get involved in the rescue. He has a vast knowledge of the demigod world and could use his wisdom and connections to track down Luke and free Percy. Also, some of the other demigods at the camp who look up to Percy might volunteer to help, like Clarisse. She may not like Percy much, but she wouldn't want a fellow demigod to be in the hands of Luke.
Annabeth might try to find a way to get to him. Their relationship is very strong, and she's not one to just leave Percy in danger. She could use her intelligence to figure out a way to enter Tartarus and help him escape.
Well, one possible plot could be that Luke kidnaps Percy to use him as a bargaining chip. Maybe he wants to trade Percy to the Titans for more power or a higher position. Luke might keep Percy in a secret location, like an old, abandoned temple. Percy, of course, would try to escape and find a way to warn his friends at Camp Half - Blood.
Annabeth could be a likely candidate. She's always been by Percy's side, and she'd stop at nothing to save him. Her intelligence and combat skills would be useful in the dangerous environment of Tartarus.
Luke in Percy Jackson fanfiction can play various roles. He could be a reformed villain, seeking to make amends for his past actions. This gives a new arc to his character that wasn't fully explored in the original books.
Percy might meet an old enemy who has somehow been resurrected or changed in the future. This enemy could be more powerful and have a new plan to take over the world or destroy the demigods. Or he could meet a group of time - traveling demigods from an even further future who are trying to fix something in Percy's future that has gone wrong.
There's a chance that it could be a character from Percy's family. For example, a distant relative who has been corrupted by some dark magic. They might betray Percy for their own gain or because they are being controlled, and this betrayal could lead to Percy being killed.
He might encounter new demigods who are also on the run or hiding from the traditional demigod society. These could be demigods with different powers or backgrounds that Percy has never seen before.
Annabeth could be the one to come to Percy's rescue. She is very intelligent and resourceful. She might figure out Gaea's weaknesses and use that knowledge to free Percy.
Another great author is 'MythologyFanatic'. They are good at incorporating other mythologies into the Percy Jackson universe in their fanfictions. When it comes to Luke - Percy stories, they often add new elements from different mythologies that make the relationship between the two characters even more interesting and complex.
Annabeth Chase would most likely be one of his allies. She has always been by Percy's side, and she shares his sense of justice. If Percy decides to take on the gods, she would be there to support him with her intelligence and strategic thinking.