Character development is another great aspect. In these crossovers, characters from both universes often have to step out of their comfort zones. Percy might have to deal with the more cut - throat and modern world of the Avengers, while the Avengers have to understand and accept the existence of magic and gods. This forces them to grow and change in new ways, which is always exciting to read about.
The character interactions. When different universes cross over, seeing how characters like Iron Man interact with, say, characters from the Star Wars universe (in some really creative crossovers) is fascinating.
The mixing of different power sets. For example, Buffy's slayer strength and combat skills combined with Thor's god - like powers make for some epic fight scenes. It's fascinating to see how they would interact in a battle situation.
The combination of powers. You get to see the superpowers of the X - Men, like Jean Grey's telekinesis and Cyclops' optic blasts, interact with the powers of the Avengers such as Thor's lightning and Captain America's super - strength. It makes for some really epic battle scenes.
The character relationships are also very unique. You can have unexpected friendships form, like between the mischievous Loki and the equally sly Uma from the Descendants. Or romances that cross the two universes, which add a whole new layer of intrigue and drama to the fanfiction.
Definitely Loki. In a 'Percy Jackson and Avengers crossover fanfiction', Loki would find Percy's world of Greek gods and demigods fascinating. He could try to use Percy's knowledge of the gods to his own advantage, or maybe even form an unlikely alliance with Percy. Loki's mischievous nature would add a whole new layer of chaos and intrigue to the crossover.
The cultural clash. Demigods come from various mythologies with their own cultures and values. Avengers have their own set of ethics in the superhero world. Seeing how they handle the differences and find common ground is fascinating. For example, a demigod's view on life and death might be very different from an Avenger's. The fanfiction can explore how they bridge this gap. It also allows for new storylines and character growth as they learn from each other.
I think the interaction between Hulk and Grover would be fascinating. Grover is this gentle, nature - loving satyr, and Hulk is, well, the big green rage - monster. Grover could try to use his empathy and nature powers to soothe the Hulk, which could lead to some really comical and heartwarming moments. Also, Natasha Romanoff and Clarisse would be an interesting duo. Natasha's stealth and combat skills combined with Clarisse's brute force and no - nonsense attitude could make for some great fight scenes and character development.
One possible plot could be that the Greek gods, noticing the powerful threats the Avengers face, send Percy Jackson and his friends to help. Percy could use his water powers to assist in putting out fires during a battle like the one in New York. Meanwhile, Thor could be intrigued by Percy's connection to Poseidon and they could exchange stories about their godly fathers.
The character interactions are often the most interesting part. For example, when characters from different Marvel groups meet, like Iron Man and Captain America from the Avengers meeting Wolverine from the X - Men. Their different personalities can lead to some really fun dialogue and tense moments.
One interesting plotline could be Deadpool trying to join the Avengers officially. He'd have to go through all sorts of tests and trials, which would be hilarious considering his unpredictable nature. The Avengers would be constantly second - guessing whether to let him in or not.