Perhaps some minor gods or spirits who thrive in chaos could be allies. For example, a god of randomness or a spirit of disorder. They would understand the nature of chaos better than most and could offer unique powers or knowledge to Percy. They might also be able to guide him through the chaotic landscapes and help him when he's in a tight spot.
He could have some primordial entities as allies. Maybe Erebus, as it is also related to the more chaotic and ancient aspects of the world. Another possible ally might be some minor chaos spirits that recognize his special status.
Perhaps some of the minor gods who are tired of the traditional power structure in Olympus could be Percy's allies. They might see his rebirth as a son of Chaos as an opportunity to shake things up. Also, any being that has a connection to the concept of change or new beginnings could potentially side with Percy. This includes spirits of innovation or even some forgotten heroes who sense that Percy is the key to a new era in the world of demigods and gods.
Annabeth could be a great ally. Her intelligence and strategic thinking would be very useful in dealing with the chaos Percy often gets into. She could help him plan his moves and figure out the motives of chaotic enemies.
A powerful chaos deity could be the main antagonist. This deity might see Percy as a threat to their plans of spreading chaos throughout the world. They could constantly send their minions to attack Percy and try to corrupt his mind.
Another possible antagonist could be a rogue demigod who is corrupted by the power of Chaos. This demigod might envy Percy's connection to Chaos and decide to turn against him. He could form his own group of followers, causing chaos (literally) in the demigod world. They would oppose Percy and his friends at every turn, believing that they are the rightful heirs to the power of Chaos.
The Titans could still be a major threat. Since Percy is now the Host of Chaos, they might see him as a new opportunity to regain power. Kronos could try to manipulate Percy's chaotic powers for his own gain, or maybe use Percy's new status to cause more chaos in the world and then step in to 'restore order' in a way that benefits the Titans.
A powerful primordial being awakened from the depths of Chaos could be a great antagonist. This being could have powers that surpass even the Olympian gods and a desire to reshape the world in its own chaotic image.
It could be an ancient and powerful Titan who has escaped from Tartarus. This Titan wants to use Lady Chaos's power to take over the world. He sees Percy as a threat because of his past victories against the Titans and his connection to Lady Chaos.
Obviously, Percy Jackson is the main character. He is the one who becomes the soldier of chaos. His character development from a hero of the Olympians to a figure in the chaos realm would be a central focus.
The Titans could be great antagonists. Even though they were defeated once, they could be making a comeback with new strategies and powers, seeing the Chaos Army as a threat to their plans of regaining dominance. Another option could be the Giants, who are also powerful and have a long - standing grudge against the demigods.