Another option for the main antagonist in a 'Percy Jackson lives a new life' fanfiction is a rogue god. This god could be angry at the other gods for some reason and decides to take it out on Percy. Since Percy is a well - known and powerful demigod, he becomes the target. The rogue god could use his divine powers in unexpected ways, and Percy would have to find a way to either defeat the god or convince him to stop his rampage.
Maybe the main antagonist could be an ancient, forgotten monster that has been awakened by Percy's new life. This monster could have a vendetta against Percy specifically or all demigods in general. It could be a really tough opponent as it has been lurking in the shadows for so long, gathering strength and knowledge. It might have some unique abilities that Percy has never faced before, which would make for an exciting conflict in the fanfiction.