After Sigma announced the 24-105mm f/4 DG OS HSM lens, next rumored lenses from Sigma will 300mm f/2.8 OS and 400mm f/2.8 OS lenses. These new sports lenses will be announced sometime in 2014, according to Canonrumors.
Egami has posted a new patent for Sigma 300mm f/2.8 lens.
According to source, these sports lenses could be in testing at 2014 Winter Olympic games. And Sigma is also working on 500mm and 800mm zoom lenses.