Captain Cold could be an interesting addition. He might see the OC as a new target or an opportunity. Maybe the OC has something that he wants, like a valuable piece of technology. Or, he could form an unlikely alliance with the OC against a greater threat. Another character would be Killer Frost. If the OC has heat - based powers, they could have an epic battle or a complex relationship with Killer Frost. And of course, Harrison Wells, in any of his incarnations, could play a significant role. He might be mentoring the OC or using them for his own secret plans.
The Pied Piper could be included. He could befriend the OC and introduce them to the seedy underbelly of the meta - human world. The OC could then use this knowledge to help The Flash. Firestorm (Ronnie Raymond or Martin Stein) is also a good choice. The OC could be involved in the creation of a new Firestorm or have a connection to the Firestorm matrix. And don't forget about Joe West. He could be the OC's father figure, guiding them and helping them make the right decisions in the dangerous world they're in.
Well, Cisco Ramon from The Flash would be a great addition. He could use his tech skills to create some new gadgets or figure out ways to counter the villains' powers. Also, Martian Manhunter from Supergirl's side. His shape - shifting and telepathic abilities would add a whole new dimension to the story.
Definitely, Cisco Ramon from Flash. He can add some tech - savvy humor and his ability to create new gadgets can be really useful in the crossover. For example, he could make a device that helps Supergirl deal with a new type of kryptonite - like threat.
There could be characters like Superman or Wonder Woman. They might be involved as mediators or as characters who offer different perspectives. Superman, with his moral compass, could try to get Flash to understand the severity of what he's done. Wonder Woman might offer her wisdom on dealing with the emotional and physical consequences for Alex. Also, characters like Batman could be there, secretly observing and planning how to prevent such incidents in the future.
Obviously, The Flash and Arrow themselves are must - includes. Their dynamic and different power sets make for great interactions. Also, Cisco Ramon from the Flash side is popular as he can bring in his tech - wizardry to help in the crossover.
There could be Peter's best friend. Maybe his friend has been encouraging Peter to stand up to Flash all this time. When Peter finally does it, his friend is right there by his side, supporting him.
One popular storyline is the crossovers where Flash and Arrow team up against a common, powerful enemy that neither could face alone. For example, a super - powered villain who can manipulate time, forcing them to combine their skills and resources. Another common plot is the exploration of their friendship. Flash's light - heartedness contrasting with Arrow's brooding nature often leads to interesting character dynamics.
Professor Xavier would be a great addition. His wisdom and leadership can provide guidance to both Nightcrawler and the OC. He can also offer insights into the nature of their relationship and the challenges they might face as a mutant - human (assuming the OC is human) couple. Another good character could be Wolverine. His rough and tough nature can contrast with Nightcrawler's more gentle and spiritual side, and he can act as a protector or a source of tough love for the OC.
Cisco Ramon from the Flash's universe would be a great addition. His ability to create new gadgets and his sense of humor would fit well in a crossover. For Danny Phantom's side, Frostbite could be included. His knowledge of the Ghost Zone and his ice - based powers could be crucial in a story where they are facing a powerful enemy. Also, including the Reverse Flash as a villain would create a lot of tension and action in the fanfiction.
Obviously, The Flash and Supergirl themselves are very popular. Their unique powers and personalities make them central to most fanfictions.
Naruto himself is often a popular character in such fanfiction. Writers might explore how he could incorporate the Black Flash into his already powerful arsenal of jutsu.