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.
Chiaki Nanami could be an excellent choice. She is the Ultimate Gamer, so if you cross her with a digital or virtual world, there are so many possibilities. She could use her gaming skills to navigate through a new digital realm, like a Matrix - like world. Her calm and intelligent nature would also make her a great character to lead the group in a crossover situation. She could figure out the rules of the new world quickly and help the others adapt.
Chitoge could also be a great focus. Her tough - exterior - but - soft - inside personality would be really fun to explore in the Naruto universe. Maybe she could learn some of Naruto's values like never giving up and it could change her perspective on her own relationships.
Ross could be really interesting. Since he's an archaeologist, in a crossover with a show like 'Indiana Jones' (in a fanfiction sense), he could go on an adventure with Indiana Jones. Ross would bring his academic knowledge while Indy would have the practical experience. Their interactions would be filled with Ross trying to over - analyze everything and Indy just wanting to get the job done.
Alucard from Hellsing would be really interesting. He has such a long and complex history, and his powers are very different from the Twilight vampires. Putting him in a situation with the Twilight characters would create a lot of drama. For example, his non -chalant attitude towards killing and his dark sense of humor would contrast greatly with the more brooding and 'good -guy' Twilight vampires.
Mercury from RWBY could be an interesting character to focus on. He has a rather morally ambiguous nature already. In a Darkwatch fanfiction, he could be tempted by the power of the Darkwatch's enemies, like the vampires or demons. His prosthetic legs could also be given some kind of dark upgrade, and we could see him struggle between his own self - interest and the greater good in this new and dangerous setting.
Naruto himself would be very interesting. His energetic and determined personality would contrast greatly with the more laid - back and technology - dependent Jetsons. He could bring a sense of adventure and action to their world, and his ninja skills would be something completely new for them to experience.