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 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.
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.
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.
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.
Well, it could be about Percy facing a new and powerful threat that even the Olympians can't handle easily. Maybe he discovers a hidden 13th Olympian who has been locked away for centuries and now is causing chaos.
Well, you could write a fanfiction where Percy loses his memory and has to rediscover his powers and his place among the demigods. Or perhaps a story about how the minor gods in the Percy Jackson universe form their own alliance because they feel overlooked by the Olympians. You could also create a fanfic that focuses on Annabeth's journey if she was separated from Percy and had to fend for herself in a dangerous situation.