Who could be the main antagonist in a 'shirou is a sekirei crossover fanfiction'?

Well, it could be about Shirou getting involved in the Sekirei battles. Maybe he uses his skills from his original story to help or compete in the Sekirei world. For example, his sword - fighting abilities might give him an edge in combat against other Sekirei or their Ashikabi.

One common theme is the clash of two different worlds. Shirou comes from the Fate universe with its own set of rules, magic, and values, while the Sekirei world has its unique system of Sekirei, Ashikabi, and battles. Another theme is adaptation. Shirou has to adapt to the new rules and environment of the Sekirei world. For example, understanding how to form bonds with Sekirei as an Ashikabi.

It could be Scar. Since he is the main villain in The Lion King, in a fanfiction where Shirou enters this world, Scar would still be the one causing trouble for the pride. He might see Shirou as a threat to his plans of taking over the kingdom.

Perhaps a rogue Sekirei who has been corrupted by the power of the Juubi's energy that has seeped into the Sekirei world. This Sekirei could start causing chaos, believing that they are now more powerful than the others and should rule over them. Naruto would then have to stop this Sekirei from causing more destruction and bring balance back to the Sekirei world.

One popular Shirou is a Sekirei crossover fanfiction might be 'Shirou's Sekirei Adventure'. In this type of crossover, Shirou's character traits from his original story, like his determination and swordsmanship skills, could blend interestingly with the Sekirei world's unique elements such as the Sekirei battles and the concept of Ashikabi. Another one could be 'Shirou in Sekirei's World'. These crossovers often explore how Shirou would adapt to the Sekirei universe and how his presence might change the existing storylines in the Sekirei world.

If it's a crossover with Alice in Wonderland, the Queen of Hearts could be the main antagonist. The Queen might see Tinkerbell as an intruder and try to behead her. Tinkerbell would have to use her wits and magic to survive in the crazy world ruled by the Queen of Hearts.

A powerful demon from another dimension could be the main antagonist. This demon could be attracted to the power that Sinbad wields in the Magi world and wants to steal it for himself. He could start causing chaos in both the world Sinbad is from and the world he's been transported to in the crossover.

A new character could be introduced as the main antagonist. For example, a being from another dimension who has been observing both Sonic's and Megaman's worlds. This being realizes that by combining the chaos energy from Sonic's world and the advanced robotics technology from Megaman's world, it can create a new, all - powerful empire. So it starts to manipulate events in both worlds to bring about this combination, while Sonic and Megaman have to stop it.

Dr. Eggman and Dr. Wily could team up. Dr. Eggman has his mechanical creations and knowledge of Sonic's world, while Dr. Wily is an expert in creating powerful robots in Mega Man's world. Together, they could be a formidable force against Sonic and Mega Man.

Thanos could be an interesting antagonist in a crossover. Imagine Dr Fate having to use his magic to try and stop Thanos from getting all the Infinity Stones. Thanos' brute force and determination would be a great contrast to Dr Fate's magical finesse.