It's hard to say for sure, but Doctor Strange could be a very powerful character if he were included. His mastery of the mystic arts could give him an edge in both the magical and space - faring aspects of the story. He could manipulate time and space in ways that would be useful in the Guardians' adventures and also hold his own against powerful wizards in the Harry Potter world.
Thanos with the Infinity Stones would be extremely powerful. However, in a fanfic that combines with Harry Potter, Voldemort might also be a strong contender. If Voldemort got his hands on some of the Infinity Stones, he could become nearly unstoppable. But he would also have to face the combined forces of the Guardians and the magical community from Harry Potter, which would make for an epic battle.
I think the meeting between Thor and Groot would be fascinating. Thor, being an Asgardian god, and Groot, a sentient tree - like creature. Thor might be initially confused by Groot's limited vocabulary ('I am Groot'), but as he gets to know him, he would see Groot's strength and loyalty. Groot could also be intrigued by Thor's lightning powers. And their combined strength in a fight against a common enemy would be a sight to behold.
Definitely Thor and Star - Lord. Thor is this god - like being from Asgard, and Star - Lord is a half - human, half - alien space adventurer. Their personalities are quite different. Thor is more noble and straightforward, while Star - Lord is a bit of a scoundrel. But they both have a sense of humor and a love for adventure. They could have some great banter while fighting side by side.
Another great pairing could be Hulk and Groot. Groot's calm and gentle nature might have an interesting effect on the Hulk. Maybe Groot could help calm the Hulk down when he's about to go on a rampage, or they could team up in a fight and Hulk could use Groot as a sort of living weapon. It would be a very unique dynamic.
Black Widow and Nebula would make for an interesting pair too. Both are strong female characters who have had a rough past. They could bond over their experiences of being in the shadow of more powerful male figures in their respective groups. And they could also team up to take on some of the more dangerous enemies, using their stealth and combat skills.
A crossover might also involve the characters' personalities clashing and then growing to understand each other. Harry Potter, with his sense of justice and loyalty, could have an interesting dynamic with Star - Lord. They might start off not seeing eye - eye but then come to respect each other's values as they face a common enemy together.
Gamora and Sakura could be a great pairing. Both are strong female characters. Gamora could teach Sakura some of her advanced combat techniques from other planets, and Sakura could show Gamora how to use medical ninjutsu. They could also bond over their experiences of growing up in tough environments and having to prove themselves.
Luther might also be a strong contender. His super strength combined with some protective spells from the Harry Potter world could make him a formidable force. For instance, if he were to learn the Shield Charm, he could use his strength to hold the shield in place while also attacking enemies.
It's hard to say for sure, but it could be Harry Potter. His ability to perform a wide variety of spells, his experience in fighting dark wizards, and his connection to the powerful magic of love could give him an edge. Also, he has the support of his friends at Hogwarts who are also very skilled in magic. In a fight against the threats in the Percy Jackson world, he could quickly adapt his magic to deal with things like monsters.
They could fit in as strange visitors from another dimension. The wizards would be both curious and cautious about them. Maybe the Guardians are there to trade some unique galactic items for magical artifacts.
Harry Potter and Star - Lord would probably have an interesting dynamic. Harry is used to a world with clear rules of magic, while Star - Lord is more of a free - spirit in the vastness of space. They might initially clash but then learn from each other. Harry could teach Star - Lord about loyalty and the importance of fighting for what's right in a more selfless way, while Star - Lord could show Harry how to be more adaptable in new and strange situations.