If it's a crossover with the Green Lantern Corps, Hal Jordan or John Stewart could be good choices. They could teach the Teen Titans about the power of the Green Lantern rings and how to use them in combination with their existing powers. Plus, the intergalactic adventures that could come from this crossover would be really exciting.
Well, if it's a crossover with the X - Men, characters like Beast would be interesting. His intelligence and physical abilities could interact well with the Teen Titans. Also, Rogue could have some really cool interactions, especially if she were to accidentally absorb one of the Titans' powers.
In a crossover with the Inhumans, Black Bolt would be a great addition. His power of destruction with just a whisper could be a huge threat or a great ally depending on the situation. And Medusa, with her long prehensile hair, could have some unique battles alongside the Teen Titans.
Robin could be a great character to include. He's the leader, so his relationship with the OC could be one of mutual respect and leadership. Starfire is also a good choice. Her bubbly and kind nature could make for an interesting dynamic with the OC. Beast Boy with his sense of humor could add some light - heartedness to the harem. Cyborg, with his technological expertise, could have a different kind of connection with the OC, like helping the OC with any tech - related problems they might have.
In a crossover with Devil May Cry, Dante would be a great addition. His cocky and powerful attitude would contrast well with the more brute - force nature of the Splatterhouse protagonist. They could team up to fight some sort of demonic invasion that threatens both of their worlds.
Interactions in this crossover would be diverse. Willy Wonka would be intrigued by the Titans' powers. He might ask them to use their abilities to help him with some new creations in the factory. For instance, he could ask Starfire to use her starbolts to heat up a special chocolate. The Oompa - Loompas could be both entertained and a bit scared of the Titans at first. They might sing songs about the Titans' strange appearances and powers. The Titans, in turn, would have to adapt to the Oompa - Loompas' unique culture within the factory and find a way to work together to have a positive experience.
John McClane from Die Hard would be a great addition. He has a similar tough - guy attitude and is good at handling high - pressure situations in urban environments. Another good character could be Ellen Ripley from Alien. Her experience with alien threats would contrast well with Rambo's combat skills. Also, Indiana Jones would be a perfect fit. His knowledge of archaeology and adventure - seeking nature combined with Rambo's combat expertise could lead to some exciting storylines.
Storm from the X - Men would be a great addition. She has control over the weather, which could create some really interesting scenarios when combined with Wolverine's combat skills. For example, she could create a lightning storm while Wolverine battles enemies in the rain, using the flashes of lightning to his advantage.
In a crossover with Hannah Montana, obviously Miley Stewart as Hannah Montana herself. She could be in Seattle for a secret concert and needs iCarly's help to promote it without revealing her identity. It would be fun to see Sam trying to keep Hannah's secret while also being her usual wild self around her.
In a crossover with the Lord of the Rings, Aragorn could be an interesting choice. He could bring his leadership skills and combat expertise to Los Santos. The GTA 5 characters might initially be skeptical of his noble ways, but they could learn from him. He could also help them fight against a new evil force that has emerged in the city, perhaps an ancient and powerful gang with dark magic - like abilities.
If it was a crossover with The Evil Within, Sebastian Castellanos would be a great character to include. His detective skills and his ability to handle the nightmarish creatures in his own game could be useful in the Outlast 2 world. He could team up with the Outlast 2 character to solve the mysteries behind the horror. Also, Pyramid Head from Silent Hill could be a terrifying addition. His presence alone would heighten the sense of dread in the Outlast 2 crossover story.
Definitely Goku. Their previous encounters in the Dragon Ball series make it a natural choice. Their power levels and different fighting styles would create great tension and action in the story.
Well, if you're doing a crossover with '101 Dalmatians', Cruella de Vil could be an interesting addition. She might see the Aristocats as a new target for her fur coats, creating a whole new conflict for the cats to face. And the Dalmatians could be allies of the Aristocats, helping them outwit Cruella.