Well, one possible plot could be Percy facing his inner demons in Tartarus. Since he's alone, he might have to rely solely on his own skills and memories. Maybe he'll encounter some long - forgotten monsters that test his strength and willpower. He could also stumble upon some hidden secrets about the place that no one has ever known before.
It could be Nyx. She is a very powerful primordial goddess in Tartarus. Since Percy is alone, she might see him as an easy target or a threat to her domain. Nyx could send all sorts of dark creatures and illusions to stop Percy from achieving whatever goal he has in Tartarus.
He might face numerous terrifying monsters. Tartarus is full of dangerous creatures in the fanfic world, and Percy would have to fight for his survival all by himself.
Bob the Titan could potentially be an ally. Since he has some knowledge of Tartarus and has shown kindness before, he might come to Percy's aid again. He could help Percy navigate the dangerous areas and avoid some of the more powerful threats in Tartarus.
One possible plot development could be Percy facing off against Tartarus' own unique and terrifying monsters all by himself. He might have to rely solely on his wits and the powers he has developed over his previous adventures. For example, he could use his water - controlling abilities in creative ways to navigate the strange and dangerous terrain of Tartarus.
There are likely some out there. You could try searching on fanfic platforms like Archive of Our Own. They have a large collection of fanfictions and you might be able to find one with this particular pairing.
One possible plot could be that Luke initially starts out as a rival to Percy but as they interact more, he begins to see Percy's true self and falls for him. Maybe they are forced to work together on a dangerous quest and during that time, Luke's feelings develop.
I also like 'Love's Awakening'. This fanfic takes a different approach. It starts with Annabeth being in denial about her feelings for Percy. But as the story progresses, through various adventures and close calls, she finally succumbs to her love for him. It's a great read for any fan of the ship.
He uses his combat skills. Percy is a skilled fighter, so he'll likely rely on his sword - fighting abilities to take on the monsters he encounters in Tartarus.
Well, in such a fanfic, Percy would probably have to dig deep into his own skills and experiences. Since he's alone, he can't count on Annabeth's strategic thinking or Grover's sensing abilities. He might have to face a god who's out to get him. He'd be forced to use his powers in creative ways, like using his water - control to escape from a labyrinth - like trap instead of having someone else to help him find an exit.