It would be interesting, but as others have said I don't see it happening. For one, what kind of plot would they go for? They already used Demon Lords, and they don't seem the crowd to require 20th level PCs to fight. Giants, another obvious option, done for 5th to 10th level characters.
A bigger problem is that most adventures start with a small railroad - captured by Drow!, on the road to Phandalin!, mists!, arriving in Red Larch! - and they can easily do that because 1st level characters are small in scale, both in their world view and their abilities. Starting a campaign at 15th level, well, what kind of a railroad will work on characters who have access to Teleport, Scry and Plane Shift?