Sonic would be really interesting. His super - speed and adventurous nature would contrast well with the Powerpuff Girls' more home - based superheroics in Townsville. His encounters with the villains in Townsville would be full of action as he zooms around trying to stop their evil plans, all while trying to figure out how to get back to his own world.
Maybe there's a device that combines the chaos emeralds' power with Chemical X. This causes a strange rift in the dimensions, bringing Sonic and the Powerpuff Girls together. They have to work together to reverse the effects of this strange combination. Sonic uses his speed to gather the scattered chaos emeralds, and the Powerpuff Girls use their powers to keep the villains from interfering.
The Powerpuff Girls Z themselves - Blossom, Bubbles, and Buttercup are often very popular. Fans love to write about their different personalities and how they interact with each other. For example, Blossom's leadership, Bubbles' kindness, and Buttercup's tough - girl attitude. Also, the villains like Mojo Jojo can be popular. His devious plans and his rivalry with the girls make for interesting stories in fanfic.
Well, it's hard to say exactly who the most popular ones are as it can vary from person to person. But some OCs that are often liked are those with really cool and unique powers. For instance, an OC with the ability to control elements like fire, water, earth and air all at once is likely to be popular. They can bring a whole new level of action to the fanfic.
Gohan could be really interesting. He has so much untapped potential. In a tournament, he could finally fully embrace his Saiyan heritage and show off new forms or techniques that we haven't seen before. He also has the added drama of trying to balance his academic life with his fighting responsibilities.
Brick from the Rowdyruff Boys might be the most interesting. He's the leader of the boys and is used to being in charge. When he meets the Powerpunk Girls, he has to deal with another group with their own hierarchy. He might struggle with how to interact with them, whether to compete or cooperate. His character development throughout the fanfic could be really fascinating as he tries to figure out his place in this new situation.
I think Twilight Sparkle would be really interesting. Her vast knowledge of magic could be used in unique ways in the XCOM universe. She could analyze alien technology and find ways to counter it with her spells. Also, her leadership skills from Equestria could be applied to XCOM operations.
Well, obviously the main characters would be the Powerpuff Girls - Blossom, Bubbles, and Buttercup. And Sonic the Hedgehog would also be a central character. There could also be other characters from both universes like Professor Utonium from the Powerpuff Girls side and Tails or Knuckles from Sonic's side depending on the direction of the fanfic.
One interesting storyline could be the Powerpuff Girls Z facing a new, super - powerful villain from another dimension. This villain has powers that are the opposite of theirs and can manipulate time, causing chaos in Townsville. Another could be a story where they lose their powers temporarily and have to rely on their wits and teamwork to save the day. Maybe they team up with some of their former enemies who also realize the new threat is too great for them to handle alone.
Butch from the Rowdyruff Boys is also quite popular. His wild and impulsive nature can lead to a lot of action - packed and humorous situations in fanfic. Writers can explore his growth from a troublemaker to someone with a bit more depth.
Gir could also be a great focus. His love for food and randomness could add a really fun element. Imagine Gir befriending the Powerpuff Girls and getting them into all sorts of silly adventures, while Zim is fuming in the background because Gir is not helping with his evil plans.