These two are my favorite crossover idea.
It was Saturday night and the turtles were preparing for their weekly movie night. April and Casey were sitting next to each other, which made Donnie mad. Mikey was in the kitchen, waiting for the popcorn to pop. Raph was trying to find Chompy with Mona. And Leo was trying to get a hold of Karai. "This is the Snake ninja, I can't come to the phone right now, leave a message," her voice mail said.
"Karai, it's Leo. You're gonna be late for movie night. Call me when you get this. Love you," he said. He left the dojo and saw that Shinigami had arrived without Karai. "Hey Shini, have you seen Karai today?" he asked.
"Sorry, she slept most of the day and left right after the sun went down. She told me that she was just going on patrol," she explained.
Leo thought for a second. "Weird, she never goes by herself on patrol, maybe we should check if everything is okay," he suggested.
Donnie came over. "Well, I do have her t-phone's GPS on mine so I could track her," the smart brother said.
April got up from her seat and said, "Guys, I'm sure that Karai is just taking some time to herself. You shouldn't be prying into her privacy," she told them in a stern voice.
Mikey came in with popcorn, t-phone in hand, and confused expression. "Hey, guys, I think Karai pocket dialed and sounds like she's in trouble," the youngest turtle said. He handed Casey the popcorn and put his phone on speaker and the sounds of a struggle came through.
Leo and Donnie looked at April with smug looks on their faces. "No one likes an 'I told you so'. Come on, let's go help her," she said as she ran to the Shellraiser.
Leo told Raph and Mona what was going on and they were off. They arrived at the coordinates that was revealed to be the abandoned Pack Media Studio building. "Isn't this the studio where they filmed that old show 'The Pack' back in the early 90's?" Casey said. "That show was so corny."
"You are going down!" They heard Karai's voice coming from the building. They all ran in and were surprised to see Karai in snake form fighting a white wolf mutant in battle armor. "Guys? What the heck is going on? How did you know I was here?" she asked as she tail slapped the wolf in the face. Before they could answer 5 gargoyles fell from the rafters and to twin cyborgs and a robot landed on their feet. "Okay, I can explain," Karai said as she turned back into human form.