Chris Perkins and Greg Tito discussed Thay and the Red Wizards recently on Dragon Talk: they sure make it sound like Thay will be a big bad in the near future:
Red Wizards appeared in a few of the D&D Next playtest adventures, which definitely made it seem as if they would be important adversaries in the future.
Personally, I love using wizards as villains. Their large arsenal of spells makes them very interesting and versatile as antagonists (both in and out of combat).
They're supposed to be the villains for the Ghosts of Saltmarsh season DDAL adventures, aren't they? I wonder if there's more to it than that.