A fallen demigod could be the main antagonist. This demigod has turned against the Olympians and wants to use the power of the ancient artifacts that Percy and Lara are seeking to challenge the gods. He has some knowledge of both the demigod world and the secrets of the tombs, so he is a very dangerous enemy. He could use his own powers to create illusions and mislead Percy and Lara, or he could summon mythical beasts to attack them while they are distracted by the challenges of the tombs.
Maybe a group of mercenaries who are also interested in the valuable artifacts related to Greek mythology. They would be willing to do anything to get their hands on them, and they are a threat not only because of their weapons but also because they have some knowledge of the ancient secrets that could be used against Percy and Lara. They could set up ambushes and try to steal the artifacts from Percy and Lara while they are in the middle of their exploration.
A possible scenario in this 'Percy Jackson meets the Tomb Raider' fanfiction is that Lara is on a quest for an ancient artifact that is related to Greek mythology. She stumbles upon Percy, who is also on a mission for the gods. They initially distrust each other but soon realize that they can achieve more by working together. Percy's demigod abilities such as his enhanced strength and speed, along with Lara's combat skills and archaeological knowledge, make them a formidable team. They could encounter challenges like traps set by ancient civilizations that require both Percy's powers and Lara's wits to overcome.
It could be a new monster that is specifically targeting Percy's mortal connections, like Paul. Maybe a monster that feeds on the fear of mortals who are close to demigods.
A great antagonist could be a rogue demigod who has been corrupted by power. This demigod could have discovered a way to combine the darkest parts of Greek and Roman magic and is using it to cause chaos among both camps. He or she might see Jason and Percy as the biggest threats to their plans and target them specifically.
There could be a new, made - up character. A digital wizard of sorts who has learned how to cross over from the popularmmos world to the Percy Jackson world. He wants to control all magic and technology, believing that by merging the two worlds under his rule, he can become the most powerful being. So he sets out to stop anyone who stands in his way, including Percy and the popularmmos crew.
Perhaps a rogue group of mermaids and mermen. They don't like the idea of Triton allying with Percy, a half - blood. They believe that Triton should only focus on the pure - blooded sea creatures. So they become the main antagonists, constantly attacking Percy and trying to turn Triton against him.
Another possible antagonist could be a powerful sorcerer or witch who is envious of Aphrodite's beauty and power. They see Percy as a way to get to Aphrodite. So they try to use Percy as a pawn, casting spells on him to make him do their bidding and cause harm to Aphrodite, all while Percy is trying to figure out what's going on and break free from the control.
It could be a new monster that is after Percy. Since Percy is a demigod, there are always monsters chasing him. This new monster might be something that Paul has never seen before and it could put both of them in danger.
It could be a powerful mage who wants to use Percy's demigod powers for his own evil plans. This mage might be jealous of Percy's unique abilities and try to control him.
It could be a character who is jealous of the power that could come from combining the two worlds. Maybe a rogue demigod who wants to use Jessie's knowledge of her world's technology and the magic of the Percy Jackson world to rule over both. This character might start causing problems by stealing important artifacts from both worlds to try and build a device that would give them ultimate power.
A possible main antagonist could be a new character created specifically for this fanfiction. A scientist who has somehow managed to combine Greek magic with Hydra technology. This character could be creating powerful hybrid creatures that are causing havoc in both Percy's world and the Marvel universe. Percy and Black Widow would need to put aside their differences and work together to stop this mad scientist.