Robin would also be fascinating. Her calm and intelligent demeanor would be a great contrast to the chaos of the games. She could use her knowledge of history and her devil fruit powers in unique ways in the magic - based challenges. For example, she might discover ancient magic spells hidden in the ruins of the game arena and use them to her advantage.
Sanji could be a great focus too. Imagine him in a cooking competition within the Grand Magic Games, but with a magical twist. He has to use magic ingredients and create dishes that not only taste good but also have magical effects. And of course, he'd be fending off any rivals who try to get in his way, especially if they're disrespectful to food or women.
Luffy would be really interesting. His carefree and adventurous nature would make his participation in the Grand Magic Games full of surprises. He'd probably approach the magic - based challenges in his own unique, unorthodox way, like trying to stretch his body to reach magic items instead of using proper spells.
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.
One idea could be that the Straw Hat crew participates in the Grand Magic Games. Luffy might see it as a new and exciting challenge, like a different kind of adventure island. He could enter all sorts of wacky competitions, like a race where they have to use magic-infused boats or something.
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.
Frisk would also be a great character to focus on. If they were to gain some of Saitama's power, it would completely change their journey in the Underground. They could use their new strength to protect the monsters instead of just trying to find a way out. This would lead to new relationships and storylines. They might be able to stand up to Flowey in a different way, and even change the way the entire Underground views humans.
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.
Ben as Diamondhead and Zoro would be really interesting. Diamondhead's crystal - like body could provide a great defense against attacks, and Zoro's swordsmanship could be enhanced with Diamondhead's sharp edges. They could team up in battles and create some unique combat strategies.
Naruto and Luffy would be a really interesting pairing. Naruto's determination and Luffy's carefree yet strong - willed nature would create a great dynamic. They could learn a lot from each other, like Naruto learning about Luffy's ability to make friends so easily and Luffy learning about Naruto's never - give - up attitude towards his goals.
Luffy and Ruby would be an interesting pairing. Luffy's carefree and determined nature combined with Ruby's innocent yet strong - willed personality would create some great interactions. They both have a sense of adventure and are leaders in their own groups.
Sasuke and Zoro could have some cool interactions. They are both very strong and somewhat brooding characters. Zoro might be interested in Sasuke's Sharingan and the power it holds, while Sasuke could be intrigued by Zoro's three - sword style.