Darth Vader would be an amazing character to include. His imposing presence and incredible power would be a huge threat in the MCU. He could face off against Thor, with their different types of powers creating an epic battle. Another interesting character would be Kylo Ren. His conflicted nature could add an interesting dynamic, especially if he interacted with characters like Captain America who have strong moral compasses.
One of the most interesting characters to include would be Palpatine. His devious and manipulative nature would fit well in the MCU. He could try to play the various factions against each other, just as he did in the Star Wars universe. Also, Doctor Strange would be a great addition. His knowledge of magic could be pitted against the dark side of the Force. The two could have a battle of wits and powers, which would be fascinating to read about in a crossover fanfiction.
Definitely Thanos and a Sith Lord would make for a great combination. Thanos with his goal of collecting the Infinity Stones and a Sith with their lust for power. They could either be allies or enemies depending on how the story goes. And Scarlet Witch could also be an important character. Her powers are somewhat similar to the Force in a way, and she could have an interesting interaction with a Sith, either being tempted by the dark side or using her powers to resist it.
Thor and Godzilla would be an interesting pair. Thor, with his god - like powers and his hammer Mjolnir, could potentially have a showdown with Godzilla. It would be a clash of different kinds of power - Thor's lightning and brute strength against Godzilla's atomic breath and massive size.
Black Widow and Monarch scientists would be an interesting pair. Black Widow is used to dealing with espionage and sneaking around. The Monarch scientists have in - depth knowledge about the Titans. She could help them in a more tactical way, like getting information from shady organizations that might be trying to use the Titans for their own gain. And the scientists could provide her with valuable information about the kaiju to help the Avengers in their fight.
Definitely Miles Morales. His different origin story and unique powers compared to MCU's Spider - Man would make for some great interactions. He could show Peter Parker from the MCU new ways to use his spider - powers.
One idea could be that a Sith Lord somehow gets transported to the MCU universe and tries to take over Hydra. They could use their dark side powers to manipulate the already corrupt organization. Another could be a story where a young Jedi in training from the Star Wars universe ends up in the MCU and teams up with the Avengers to fight a Sith who has also made their way there. Maybe they have to combine the Force with the superpowers of the Avengers to defeat the Sith.
Anthony Bridgerton would be a great character to include. His strong sense of family and his often stubborn nature could lead to some interesting clashes in a crossover. Say, in a crossover with a fairytale world like Cinderella. He might initially be against the idea of the Bridgerton family getting involved with the magic and 'foolishness' of the fairytale world, but then gradually change his mind.
Definitely Nigel Marven from 'Prehistoric Park'. His knowledge and enthusiasm for prehistoric animals would make him a key figure in any crossover. If it was a crossover with 'Terra Nova', he could team up with the Shannon family. His expertise would help them better understand the prehistoric world they are living in and he could also learn from their experiences in a colony in a prehistoric environment.
The Prophet of Truth from Halo could also be an interesting addition. His devious plans and manipulative ways could create a lot of chaos in the Fallout universe as he tries to gain power over the different factions and their resources.
Ash Ketchum would be a must - include. Imagine him using his knowledge of Pokemon to master Beyblade battles. Also, Gary Oak could be his rival in this new form of combat. From the Beyblade side, Tyson could be an interesting addition. He could bring his Beyblade skills and learn about Pokemon. And of course, Pikachu as a character - like presence in the form of a Pikachu - themed Beyblade would be really cool.
Definitely Regina from OUAT. Her complex past as the Evil Queen and her journey towards redemption would be fascinating in the context of the 'Descendants' world. She could interact with the villain kids and perhaps offer them some wisdom from her own experiences of trying to break free from her dark past.
For the PJO side, Nico di Angelo could be a great character. His control over the dead could have some really strange and interesting interactions in Skyrim, where there are also undead creatures. On the Skyrim side, Ulfric Stormcloak or Elisif the Fair could be included. Elisif's leadership and Ulfric's military might could be factors when dealing with the demigods. If the demigods get involved in the civil war in Skyrim, their powers could completely change the balance of power. And Luke Castellan from PJO could also be an interesting character to bring in, as his complex personality could add a lot of drama to the crossover story.