In his reddit AMA today Mike Mearls was asked by JesterRaiin “And now, the question: could you confirm both [Dark Sun] and Ravenloft coming back?“
Mike Mearls responded:
“I can’t confirm anything, but I can say that we’re big fans of the classic D&D settings. When we created the core 5th system, we new that it had to be flexible enough to both cover past D&D settings and have the ability to stretch to cover new ones.”
So have faith Dark Sun and Ravenloft fans. You have not heard a solid “No” yet.
Here’s a link to the Mike Mearls AMA on /rpg