A powerful and corrupted former Kamen Rider could be a great antagonist. Maybe someone who was once a hero but was consumed by power and now wants to take over the world using their Rider abilities for evil. They would know all the tricks of the trade and be a formidable foe for Phantom.
A mysterious and shadowy organization within Phantom's own world could be the antagonist. They operate in the shadows, pulling the strings and creating chaos. They have agents everywhere and are constantly trying to undermine Phantom's efforts to protect the city. Their motives could be related to some dark secret or power that they believe will be unleashed if Phantom is successful in his duties.
An interesting antagonist for a crossover could also be an evil Kamen Rider from a parallel universe. Let's say there's a version of a well - known Kamen Rider who has turned to the dark side. This evil Rider could be jealous of Chalice's powers and wants to take them for himself. He could be forming alliances with other villains from different Kamen Rider series in order to achieve his goal, making him a formidable and complex antagonist in the crossover story.
Sam Manson would be an excellent addition. Her strong - willed and goth - like nature can add an interesting dynamic. From the Kamen Rider side, if we consider Kamen Rider W, Shotaro Hidari could be included. His detective skills combined with Danny's ghost - hunting abilities would make for some great mystery - solving scenes. And let's not forget Jazz Fenton, Danny's sister. She could provide some emotional support and also get involved in the action in her own way.
Super Sentai characters would be great. For instance, a member of a Super Sentai team could team up with Zero One. They could exchange fighting techniques and learn from each other. The Sentai member might teach Zero One about teamwork in a large group, while Zero One could show the Sentai member some advanced technology - based attacks.
Another Kamen Rider from the same series who has a different view on justice could be an ally. They might see that Cronus's power could be used for good if guided in the right direction.
One idea could be a story where Kamen Rider Phantom has to team up with an unexpected ally from a different dimension. This ally could bring unique powers and a different perspective, creating new challenges and adventures for Phantom.
One interesting story could be about Danny Phantom getting the powers of a Kamen Rider and using them to fight supernatural threats in both his world and the Kamen Rider world. He might team up with a Kamen Rider to take on a powerful ghost - demon hybrid villain.
Another possible antagonist could be the new opera managers. They could be trying to use Christine's disappearance to their advantage, perhaps by spreading rumors that she has run away with a lover to create a scandal and boost ticket sales. They might also try to stop Raoul from searching for her, seeing it as bad for business.
Another antagonist could be a group of Kamen Riders from different timelines who see Dark Decade as a threat to the stability of the multiverse. They might band together to stop him, even if he has good intentions in the fanfiction's plot.
In my opinion, Ultraman could be a great character to crossover with Kamen Rider Agito. They both have their own unique powers and fighting styles. Ultraman's large - scale battles against giant monsters could contrast with Agito's more street - level combat against the Unknown. And they could combine their powers in some creative ways, like Agito using his form - changing abilities to adapt to Ultraman's size when they need to fight a really huge enemy together.
It could be one of the well - known kamen riders like Kamen Rider Decade. Since he has the ability to travel through different worlds, it would be plausible for him to end up in the world of Highschool DxD. He could use his powers to adapt to the new environment and deal with the various challenges there.