A 24-70mm f/2.8 DG OS Art might be the next lens to join the full-frame camp. I see it coming before a 85mm f/1.4 DG Art or 135mm f/2 DG Art.
A macro lens is likely as well, but things are fuzzier here and we could speculate endlessly. We have three medium and long lenses from the EX era (105mm, 150mm and 180mm f/2.8 EX), two discontinued lenses (50mm and 70mm f/2.8 EX) due to “not so much demand“, and Sigma’s CEO stating that a new macro glass is one of the top user requests. So, from an update to something completely new — everything is possible. I’m leaning towards a new 105mm f/2.8 DG OS Macro, but I wouldn’t write it in stone.
We can also expect a 70-300mm f/4-5.6 DG OS Contemporary at some point. Those affordable telezoom lenses are very popular. The current Sigma models are on the shoddy side, and don’t quite make it with today’s sensors.