The rumor mill is turning very slowly.
Sigma is used to announce something in the springtime, and the 50/1.4 Art price and availability may have played this role. Should another spring or early summer announcement occur, I would place my bet on a Contemporary telezoom lens.
Many people are yearning for more of these delightful Art primes (see the related poll), but they are long and hard to design, so we should be over for quite some time.
Looking further, Photokina should be interesting: Sigma SD1 Quattro, 24mm f/1.4 Art and super telephoto Sports lenses (no certainty on these, but likely), and possibly more.
Fuji Rumors is reporting about possible X-mount lenses introduced for Photokina. Sigma said back in December that they had no plans for this mount. This can change given the success of Fuji’s X series of cameras and Sigma’s will to support as many mounts as possible, but I personally think it will take a bit more time. Let’s hope I will be proven wrong.
Besides, we may see upgrades in the “accessories” area, but they will be difficult to predict. For instance, the two teleconverters and the EF-610 DG flashguns would benefit from an update. There is room for a quality bump here.