One possibility is that Percy forms really deep friendships with the gods, not just as allies. He could learn about their personal stories and secrets. This could lead to some interesting situations, like when Percy has to mediate between two quarreling gods because of his new - found friendship with both of them.
Well, there could be Percy going on quests with the gods by his side. Maybe they help him face new and more powerful monsters that he couldn't handle alone. For example, fighting a giant that has been awakened by an ancient curse.
One possible plot could be that Percy and his friends are faced with a new and extremely powerful threat that endangers not only the mortal world but also the entire pantheon. In order to defeat it, they are offered a chance to become gods. They have to go through a series of trials that test their loyalty, courage, and wisdom even more than before. Percy might struggle with the idea of leaving his mortal life and family behind, but realizes it's for the greater good. Along the way, they face old enemies who are jealous of their potential ascension and try to stop them at all costs.
Athena might also be a good candidate. Although she and Poseidon have their differences, she could see Percy's potential and befriend him. She could offer him wisdom and strategic advice for his various adventures.
He could go on adventures to the far reaches of the Underworld. With all the gods' powers, he might be able to access areas that were previously off - limits. He could also journey to the homes of the gods and goddesses to learn more about their powers and how to use them better.
Maybe Percy unknowingly breaks some ancient and very strict laws of the Gods. For example, he might interfere with some sacred rituals or places that are off - limits to mortals, even demigods like him. In the fanfiction world, the Gods could be very unforgiving when it comes to such violations and decide to kill him as a result.
Well, he could encounter unique mythical creatures that are specific to the woods. Maybe he'll find a hidden grove protected by dryads where he has to prove himself worthy to enter. It could also be that he discovers an ancient magic in the woods that helps him in his quests.
As Praetor, Percy could be involved in diplomatic relations within the Roman demigod community. He may have to make tough decisions about alliances and resource distribution. Also, his relationship with Annabeth might face new challenges due to the different Roman and Greek cultures. For example, there could be rules in the Roman camp that conflict with their relationship as it was in the Greek world. Fanfiction often explores how they overcome these cultural differences while Percy fulfills his duties as Praetor.
He could be involved in some really cool quests. Like finding some ancient artifact that's hidden deep within Olympus. He'd probably make new friends among the minor gods and goddesses. And of course, there would be the constant threat of some ancient evil trying to take over Olympus, and he'd be right in the middle of the fight to defend it.
He might discover new sea creatures that are unique to Atlantis. Maybe he'll find a hidden school of magic there, or encounter ancient sea gods who have been forgotten by the outside world.
Grover might be the most likely to resist. He has always been very connected to the mortal and natural world. His role as a satyr has been to protect and guide demigods in the mortal world. Becoming a god would mean leaving behind his familiar role and the simple joys of the natural world he loves so much. He might fear losing his identity and his close connection to the forests, meadows, and the young demigods he has befriended over the years.
In this fanfiction, the plot might revolve around Percy's struggle as a slave of the gods. He could be used as a tool by the gods in their power games. For example, the gods might send him to retrieve powerful artifacts from dangerous places, and all the while he is trying to resist this unjust treatment and regain his freedom.