For a while, Paramount was working on an adaptation of Teenage Mutant Ninja Turtles: The Last Ronin, a story that followed the last surviving member of the TMNT gang, as they exact revenge against those who slayed its brothers and brought New York City under its heel. It was supposed to be a gritty and violent live-action project headed up by Nobody creator Ilya Naishuller, but it has since been reported that this film is no longer being focused on by the studio.
As per The Hollywood Reporter, Paramount has iced the project because of concerns that the first live-action TMNT in a decade being a violent R-rated movie won't go down well with wider fans. Instead, it is changing direction and working on a more accessible alternative that will be better for a wider age demographic. This doesn't mean that the film won't be made, as it's noted that the door is being left open to a possible revisit in the future.
This is part of Paramount's plan to continue to invest in the TMNT world, as on top of the sequel to Mutant Mayhem coming out in 2027, there are claims that the studio wants to "Sonic-fy" the Ninja Turtles and create a similar universe to what Paramount has already achieved with Sega's mascot, Sonic the Hedgehog, in cinemas.