Well, there are often fanfictions where Markiplier teams up with a whole bunch of different characters. You could look on FanFiction.net too. Sometimes the stories are comical, for example, Markiplier getting into all sorts of wacky situations with his friends and fellow content creators. They can be really fun to read.
Sure. One popular one might be on Archive of Our Own. You can search there using the tags 'Markiplier' and 'everyone' to find various stories. Some might be about his adventures with other YouTubers in a fictional setting, like them all being in a video game world together.
A great place to start looking for Markiplier x Everyone fanfiction is Tumblr. There are often fic recs (fanfiction recommendations) posted by fans. Some of these fics might focus on Markiplier's relationships with his community, like how he interacts with his fans in a fictional story. It could be about a big event where everyone is involved, and Markiplier is at the center of it all.
Chapter 659, "Bodyguard Agreement", is the clearest early answer because a shaky connection becomes usable in the plot. The chapter turns that into something concrete by moving from Besides, how easy is it to get into the Gu Family? to The woman who had defeated him and taken all his hopes..
Chapter 261, "They've Ruined the Entertainment Industry!", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event. A useful checkpoint is the way the scene starts with The village chief's eyes nearly popped out after he heard the whole...
Chapter 470, "Why Talk About Pigs When Looking at Him?", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. Early on, Wrapped in a bit of warmth, Jiang Yan slowly came to..
Chapter 303, "Child Disobedient? Pull Down Their Pants and Spank Them Hard!", is the clearest early answer because it turns Zhou Wenqi's situation into a concrete plot event.
Chapter 472, "The Stream Dried Up?!", is the clearest early answer because it turns Zhuola's situation into a concrete plot event. Early on, Guan Xiong and his team had not had a proper meal in almost two or.... By the end, Zhuola was the earliest to wake up..
Chapter 664, "Record a Major Mistake", is the clearest early answer because it turns Ling Bei's situation into a concrete plot event. The setup is Shiguang and Xi Yang are fighting?, but the pressure keeps building through This is so explosive??] [Why are they hitting our poor little....
Chapter 824, "Qixing Tower", is the clearest early answer because it turns Yu's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Just as he was feeling alarmed and drenched in sweat, Mr.
Chapter 366, "The Safety Lock Got Flattened?!", is the clearest early answer because it turns Jiang Yan's situation into a concrete plot event. The chapter gives readers a real answer by opening on You're finally out!" Xiao Wu saw Jiang Li and the others from a...
Chapter 320, "How is She Here Too?!", is the clearest early answer because it turns Xiao Su's situation into a concrete plot event. The chapter turns that into something concrete by moving from Although she thought this way, Su Yinwan still knew the importance of...
Chapter 352, "Furong Cake", is the clearest early answer because it turns Lin Man Ru's situation into a concrete plot event. It opens with Finding Liu Suqing's address was not an easy task. and closes on It was filled with various flowers and plants, fragrant and....