Not a big fan of the Ronin stuff personally either, the TMNT to me are too much of a "positive" force despite how the original books were quite dark. I think if they did it as an elseworld's turtle film, like something where a character had to prevent that future from happening, or all 4 brothers had to prevent the events that brought it about like Terminator 2 somewhat it could please both fans if the first film (done as 2 films) were the actual Ronin storyline and at the end the 90s turtles are reintroduced as a means of stopping that particular future which would make up the second film. Time Traveling Casey Jones, Astral Projection Splinter, make up whatever you want to make it cohesive.