I think people are getting too excited with the "Shared Universe" popularity created by the MCU and don't realize
it's not that easy. If the reboot TMNT movies couldn't even get to a third entry why would they try to make a universe out of it?
Also people are mistaking why it worked for say Marvel, if you think about it all these superheroes were their own comic series' and eventually they had crossovers, same with DC or say the Universal Monsters. You had many A-listers who could handle their own book later set in the same universe. TMNT is the only "A-lister", anyone else getting a spin-off (as cool as a Casey Jones movie would be) does not make it a "cinematic universe" it would just be a spin-off that have been popular way before Marvel even started making movies. So the question is more of a "should TMNT have spin-off movies?" well I'd love it but if the main heroes can't hit it big the minor characters certainly won't.