Inklings from Splatoon could also be fun to include. The Inklings' ink - based attacks and their unique culture could add a new dimension to the story. They could team up with Joe to stop an enemy that is trying to cover both their worlds in some sort of dark, gooey substance. Joe would have to figure out how to work with the Inklings' different combat style.
How about a crossover with Sonic the Hedgehog? Picture this: Sonic is running through a strange new zone and accidentally enters Joe's world. They join forces to stop Dr. Eggman who has somehow teamed up with one of Joe's enemies. Sonic's speed and Joe's Viewtiful powers would create a really fast - paced and exciting story.
Joe would probably be very confused at first in the School DxD world. He's used to his own kind of battles, and suddenly being in a world full of devils, angels, and other mythical creatures would be a shock. He might be very cautious and try to figure out who the good guys and bad guys are. The devils in School DxD might be intrigued by Joe's powers and either try to recruit him or study him. The angels, on the other hand, might be more suspicious and see him as a potential threat at first.
A great plot could involve a powerful enemy that has the ability to drain the 'viewtiful' energy from Joe. Tsukune and his friends notice the weakened Joe and decide to help. They all train together, combining their different powers and skills. The story could then build up to an epic showdown where they all have to use their combined strength to defeat the enemy and Joe regains his full 'viewtiful' power.
One creative plot twist could be that Viewtiful Joe's VFX powers start to malfunction in the School DxD world. This could lead to all kinds of chaos as he tries to control his powers while dealing with the devils and other supernatural beings in School DxD. Another could be that the characters from School DxD gain some of Joe's powers temporarily and have to learn how to use them in their own battles, which would completely change their fighting styles.
The interaction between Viewtiful Joe and Moka would be really interesting. Moka's vampire powers and Joe's 'viewtiful' abilities could create some really cool fight scenes. Also, their different personalities - Moka's shy - vampiric duality and Joe's confident heroism - would make for some great dialogue and character development moments.
The theme of identity also comes up a lot. Joe Fixit has to figure out who he is in this new world he's been thrust into. Is he still the same tough guy, or does he have to change to fit in? In some fanfictions, he might struggle with maintaining his own identity while also trying to make sense of the new rules and values of the crossover universe. This exploration of self can add a lot of depth to the story.
Another interesting concept is a character - based crossover. A Joe like Snake Eyes could form an unlikely bond with a Transformer like Bumblebee. Snake Eyes' ninja skills could be used in combination with Bumblebee's speed and agility in combat. They might start off not trusting each other but gradually learn to work together as they face various threats.
There's also 'Joe Fixit in the DC Universe'. This fanfiction explores what would happen if Joe Fixit found himself in the world of DC superheroes. It's full of action - packed scenes as he tries to find his place among the likes of Superman and Batman. The writer does a great job of showing how his character would adapt to this new and different environment.
One common theme is the idea of unity. Different groups, like GI Joe and other fictional teams, come together to fight against Cobra. This shows how cooperation can overcome a powerful threat.