A summary with the probability of upcoming Sigma lenses has appeared on sigma-rumors.com. Most of the lenses mentioned have been written about here in the past. Sigma's focus over the last 2 years has been on high quality and affordable offerings, there's no reason to think that won't continue.
The most likely lenses to appear next would be:
- 24mm f/1.4 DG ART
- 85mm f/1.4 DG ART
Both of these lenses have a chance to appear at Photokina in a couple of months. I'd say the 85mm f/1.4 is a given, but the 24mm f/1.4 could fall to later this year or early 2015. With Zeiss expected to announce their Otus 85mm f/1.4 in September, Sigma could play spoiler with an optically great lens with autofocus at a much cheaper price.
The other lenses according to the site with the highest probability of happening are:
- 24-70mm f/2 DG ART
- 300-600mm f/? OS Sport
We've heard about both of these in the past, and I'm inclined to believe the supertelephoto zoom is definitely something Sigma is developing. The 24-70mm f/2, I'm not 100% sure on. If Sigma wants to differentiate themselves from the Canon and Nikon offerings, then such a lens would do the trick.