Well, perhaps Percy has to go through a series of trials. He might need to prove his worthiness to the Olympians. Maybe he has to battle some mythical creatures that are threatening the Olympian realm. And in the end, he gains the recognition and powers of an Olympian.
In this fanfiction, it could be that Percy is chosen for a special task by the existing Olympians. They see his potential not just as a hero but as someone who could join their ranks. So he gets trained in Olympian magic and lore, which is very different from what he's used to. He has to learn how to control new powers and also deal with the politics among the Olympians.
The Titans could be the main antagonists. They might oppose Percy's ascension to Olympian status because they see it as a threat to their own plans of overthrowing the Olympians.
The Titans might still be a threat. Even though they were defeated before, they could have some remnants or followers who don't want Percy to become an Olympian because it would strengthen the Olympian side even more.
In this kind of fanfiction, one possible plot could be Percy having to deal with the politics among the Olympian gods. He might try to bring about changes, like making the gods more involved in mortal affairs in a positive way. For example, he could convince them to be more proactive in protecting the environment which is being damaged by humans. Also, Percy would probably have to face new enemies. Since he's now an Olympian, he could attract the jealousy of some minor gods or ancient monsters who see him as a threat to the established order.
It could be an ancient deity that was banished long ago. Maybe a primordial god or goddess who was considered too powerful or too dangerous for the other Olympians. For example, it could be Erebus, the god of darkness.
His sense of humor might change. While Percy had a very down - to - earth and sarcastic humor as a mortal, as a god, he might develop a more refined and sometimes cutting sense of humor. He could use it to put other gods in their place or to make a point during a council meeting. Also, he would likely become more confident, bordering on arrogance at times. His new powers and status would give him a boost in self - assurance, but his friends from his mortal days would probably be the ones to keep him in check and remind him not to get too full of himself.
One popular theme could be Percy's adventures within the Olympic realm. Maybe he discovers new powers or meets unique Olympic deities. Another theme might be his relationships with other characters in this fanfiction's take on the Olympian world, like forming alliances or facing rivalries.
Well, Percy is obviously one of the main characters since the title mentions him. Other likely characters could be the Olympian gods and goddesses. For example, Zeus, Poseidon, and Athena are important figures in Greek mythology and would probably play significant roles in a story set in the Olympian world.