In reality, the Rat King story is more of a myth or legend. There's no concrete evidence to prove its truthfulness. It's often used in various forms of storytelling for entertainment and imagination.
The story of the Rat King often involves a group of rats forming a powerful alliance or having some sort of mysterious and mythical connection. But the specifics can vary depending on the version.
One idea could be exploring the relationship between the lone rat and the cubs in more depth. Maybe show how the lone rat teaches the cubs survival skills.
The Shredder could be a great main villain. He's always been a big threat to the Turtles and could pose an even greater danger to the cubs and the lone rat.
Another plot could be that the lone rat discovers a long - lost relative of the cubs. This relative could bring both hope and trouble. The relative might have knowledge that could help the cubs grow stronger, but also attract unwanted attention from the rat's old enemies.
I'm not sure exactly as I haven't read it in detail. But generally, it might be about a rat that has some sort of king - like status among other rats, perhaps dealing with power struggles, survival, and the unique world of rats.
I don't know exactly as I haven't read it. But it might be about a character named King Rat. It could involve his adventures, his relationships, or his struggles perhaps.