We've been told Sigma's next Art series lens will be a zoom and most likely a 24-70mm f/2.8 DG OS Art.
The last Art series zoom from Sigma was the 24-105mm f/4 OS DG Art, which has been out of production for some time while Sigma put their manufacturing capacity into the new art series primes as well as the 150-600mm Sport and Contemporary lenses, we're told production of the 24-105 f/4 DG OS Art will start again soon if it hasn't already.
There was a rumor a while ago about Sigma attempting a 24-70 f/2 lens for full frame, but we haven't heard anything about such a lens in quite some time.
You can read more speculation about Sigma's roadmap at Sigma Rumors.