I think the major problem here is that Henson needs to remind a potential audience of its glory days and staple brands that no longer contain Muppets. So I can see why Henson was trying to get Dark Crystal and Fraggle Rock projects off the ground, all the while not being able to get anything else new out there that isn't a preschool show. And even then, that was tricky.
Labyrinth is a cult movie. The thing flopped hard when it first came out, but gained a following. Trying to purposely make a cult movie..well... best I can say is that it has strange and varied results. A second Labyrinth film, prequel, sequel, reboot, deboot, spinoff universe... whatever it could be they'd be trying to make a follow up film to something that wasn't successful until years later. I'm not saying this isn't a good idea, but hypothetically if this actually does get off the ground, what are they going to accomplish?