We're told that Sigma is working on an Art series ultra wide angle lens. It's unknown if this would be a replacement to the Sigma 12-24mm f/4.5-5.6 DG, or an addition to the lineup. With the cost of Canon's latest EF 11-24mm f/4L, I think it's safe to assume there would be a market for less expensive version from Sigma with the optical performance of previous Art series lenses.
If true, I wouldn't expect it to be coming in the near future.
On a side note, we're told Sigma will restart production of the 24-105mm f/4 DG OS later in 2015.