[leolao]

I'd do a trilogy.

1st Movie: A sewer battle. Main villain: Rat King; 2nd villain: Baxter Stockman with robot mousers.
Post-Credits Scene: Krang arriving to earth with the Technodrome.

2nd Movie: A Technodrome Battle. Main villain: Krang with robot body; 2nd villain: Raphael brainwashed by Krang leading the foot clan. Casey Jones takes his place with the turtles, there is a Raph vs Casey fight included.
Post-Credits Scene: Bebop and Rocksteady arriving on the mutant module, kidnap April and take her to Dimension X.

3rd Movie: A Dimension X/ NYC battle. Main Villain: Shredder; 2nd villains: Slash with bebop and Rocksteady. Turtles rescue April and escape from Dimension X, villains got trapped at some prisons and Shredder disappear.
Alternative ending: Splinter sacrifices himself and die with Shredder.
Post-Credits Scene: General Traag and Granitor looking at the earth from a spaceship. Traag says: It's time!