In fanfiction, Spike could start off as a bit of a misfit in the Powerpuff Girls' universe. However, his loyalty and determination, which are traits he has from MLP, would soon earn him the respect of the girls. He could end up living in their city and helping them out in various ways. For instance, he could use his claws to climb up tall buildings to rescue people during disasters, or his fire - breathing ability could be used to melt ice during a villain - caused freeze. His presence would add a whole new layer of adventure and friendship to the Powerpuff Girls' world.
He could fit in by being a source of different magic. Since he comes from Equestria, his magic could be different from what the Powerpuff Girls are used to. This could be used to counter some new threats in their universe. For example, if there's a villain who can nullify the girls' powers, Spike's magic could be the key to defeating them. He would also add a new dynamic to the group with his unique personality.
Well, it could be a really interesting crossover. Spike could find himself in the world of the Powerpuff Girls. Maybe he'd be a bit confused at first, seeing all the different things. He might end up teaming up with the girls to fight some common enemy, like a new villain that combines the chaos of Equestria and Townsville.
Fives could fit in as a kind of protector. His skills and experience would make him a valuable asset to the ponies. He could be like a guardian, watching over the more vulnerable ponies and using his combat abilities when danger arises.
If Dib from Invader Zim were to meet the Powerpuff Girls, he'd be excited to find others who fight against the 'weird' and 'evil' forces. He'd probably try to convince them to help him expose Zim. The Powerpuff Girls might be hesitant at first, but if Dib shows them enough evidence of Zim's evil plans, they could team up.
Maybe she has psychic powers such as telepathy and telekinesis. This would be different from the Powerpuff Girls' more physical - based powers. She could use her telepathy to read the minds of the villains and figure out their next moves, while the Powerpuff Girls are busy with the physical combat part.
Finn would likely be a very determined participant. His strong sense of justice would make him oppose the killing game wholeheartedly. He might try to find a way to escape and take as many people with him as possible. Jake, on the other hand, could use his shape - shifting abilities to protect himself and others. He could turn into a shield or a large beast to fend off attackers.
Morticia could be a vampire in this fanfiction mash - up. Her alluring and mysterious nature would fit right in with the Twilight vampires. She could have her own coven, separate from the Cullens, and perhaps there would be a rivalry or an alliance formed between them based on her interactions with Carlisle. She would bring her own sense of style and elegance to the vampire world, along with the Addams family's penchant for the macabre.
Harry Potter himself could be seen as a kind of foreign wizard in the RWBY world. He might be initially confused by the concept of Dust but quickly adapt and use his magic to enhance the RWBY team's capabilities. Hermione would probably be fascinated by the study of Dust and Semblances, and use her vast knowledge to help bridge the gap between the two magic systems. Ron could be the one who forms quick friendships with the RWBY characters, sharing his sense of humor and loyalty.
Ash could fit into the Teen Titans' universe as a unique addition. He might initially be a bit of an outcast because of his different methods and his focus on Pokemon. But his determination and kind heart would soon earn him the respect of the Titans. For instance, when facing a threat that requires quick thinking and adaptability, Ash's experience in various Pokemon battles would come in handy.
Superman could fit in as an outsider at first. He'd be this super - powerful being that the Marvel characters don't really understand. His powers might be seen as a threat initially. But over time, he could become an ally. For example, he could help the Avengers during a major crisis, like an invasion from another dimension. His flight and heat - vision would be useful assets.
Well, Vegeta could be a sort of anti - hero in the DCU. His aggressive nature and desire for power could put him at odds with some of the more morally upright heroes like Superman. But at the same time, his strength would be respected. He might form an uneasy alliance with characters like Lex Luthor who also have an interest in power, though their goals would be different.