Definitely. TMNT started as a comic. Its success in the comic world led to its adaptation into various forms of media. The comic version was the original source that sparked the franchise.
You could write a fanfic where the Teenage Mutant Ninja Turtles discover a hidden underground city beneath New York. In this city, there are mutants with different abilities and cultures. The turtles have to learn to navigate this new society while also facing a threat that wants to use the underground city's resources for evil. Or, a fanfic that focuses on the turtles' training. What if they had a new mentor who taught them a completely different style of martial arts? It would change the way they fight and interact with their enemies. Also, a story about the turtles getting shrunk to a tiny size and having to survive in a world that's now giant to them could be really interesting.
The origin of the Teenage Mutant Ninja Turtles is really interesting. There was this mutagen that was created. Four turtles happened to come into contact with it in the sewers. Along with them, a rat was also affected by the mutagen. This rat, Splinter, had been observing the ways of ninjutsu from his owner before the mutation. After they all mutated, Splinter taught the turtles ninjutsu. They grew up in the sewers and became the Teenage Mutant Ninja Turtles we know. Their existence was soon discovered by April O'Neil, and they got involved in various battles against the evil forces in New York, especially the Foot Clan led by the menacing Shredder.
The availability of Teenage Mutant Ninja Turtles comics online varies. Sometimes you can find them on official comic publishers' websites, while other times they might be on third-party platforms. Just need to do a bit of searching.
One idea could be a story where the Teenage Mutant Ninja Turtles discover a hidden mecha technology in the sewers. They have to figure out how to use it to defeat a new, powerful enemy. Maybe it gives them new abilities like enhanced speed or the power to fly.