Definitely Batman and Luffy. Luffy's carefree and positive attitude would contrast with Batman's brooding nature. But they both have a strong sense of justice. Luffy punches bad guys, and Batman uses his gadgets and martial arts. They could learn a lot from each other. Another interesting pair could be Robin and Alfred. Alfred's wisdom and knowledge could help Robin understand more about the history and culture in Batman's world, while Robin could share her knowledge of the Poneglyphs with Alfred.
Luffy and Goku would be an amazing combination. Luffy's carefree and determined nature combined with Goku's love for fighting and protecting the innocent could lead to some really fun and exciting adventures. They could learn from each other's fighting styles, with Luffy being inspired by Goku's Ki - based attacks and Goku being intrigued by Luffy's Devil Fruit abilities.
I think Thor and Carlisle would be really interesting. Thor is a god with all his power and Carlisle is an old - school vampire with a strong moral code. They could have some deep conversations about life, death, and power. And then there's Jacob and Bruce Banner. Jacob's hot - headedness and Bruce's struggle with his inner Hulk could lead to some intense situations. Maybe Jacob could help Bruce control the Hulk in some way, or they could get into a big fight that the others have to break up.
I think Nami could be fascinating. In a DxD setting, she could use her wits and charm to manipulate some of the Devils and Angels, just like she does with pirates in One Piece. Also, her knowledge of navigation and treasure hunting could give her a unique role in the DxD universe, perhaps leading her to discover some hidden treasures related to the different factions.
Emma and Naruto would be an interesting combination. Emma's leadership and determination combined with Naruto's power and optimism could lead to some great adventures. They could both inspire those around them and work towards a common goal, whether it's saving the kids in 'The Promised Neverland' or protecting the village in 'Naruto'.
Aphmau from MyStreet and Freddy Fazbear would be an interesting combination. Aphmau is a strong - willed leader in her world, and seeing her face off against Freddy, the iconic animatronic from FNAF, would be exciting. Another interesting pair could be Aaron from MyStreet and Foxy. Aaron is often protective, and Foxy has that unpredictable nature.
Well, for one, Harrison Wells from Flashpoint and Castiel would be really interesting. Wells is a brilliant scientist, and Castiel is an angel. Wells could be intrigued by Castiel's otherworldly nature and try to study him, while Castiel could use Wells' knowledge to better understand the science - based threats in the Flashpoint universe. Another cool combination would be Felicity Smoak from Flashpoint and Rowena from Supernatural. Felicity is a tech - genius, and Rowena is a powerful witch. They could either be rivals, with Felicity trying to use her tech to counter Rowena's magic, or they could team up and create some really powerful spells or gadgets. And then there's Oliver Queen from Flashpoint and Sam Winchester. Oliver is a skilled archer and fighter, and Sam is a hunter. They could share their combat techniques and knowledge, and together they could take on some really tough opponents in both the Flashpoint and Supernatural worlds.
Definitely Superman and Thor. They both have super strength and are associated with powerful, other - worldly elements. Thor has his hammer and lightning powers, while Superman has his super - strength, heat vision, etc. It would be interesting to see them compete in a friendly strength - based challenge or fight side by side against a really strong foe.
Ruby and Dallas would be really interesting. Ruby's energetic and somewhat naive nature could contrast well with Dallas' more grizzled and experienced personality. They could learn from each other during heists, with Ruby teaching Dallas about the power of Semblances and Dallas showing Ruby some of the more underhanded tricks of the trade.
Another interesting interaction could be between Gwen (from Ben 10) and Nami (from One Piece). Gwen's intelligence and magical abilities would contrast with Nami's skills in navigation and her love for treasure. They could exchange knowledge, with Gwen teaching Nami some basic magic spells in exchange for Nami teaching Gwen about navigation techniques across the Grand Line.
It could be Brook. A skeleton in the Avatar world? He'd be a total anomaly. His music might have strange effects on the bending elements. He could also tell the Avatar characters stories about his past life and his adventures with the Straw Hats. The Avatar characters would be both fascinated and a little creeped out by him.