According to latest rumors on CR, Sigma is rumored to make several RF mount mirrorless lenses for Canon EOS R system. These lenses will exclusively designed for RF mount, I think it will be similar to current Sigma DG DN E-mount and L-mount lenses.
The source said first Sigma RF mount lenses will be shipping in late 2020 to 2021. The Canon’s upcoming EOS R5 has “accelerated” SIGMA’s desire to get RF mount lenses. Stay tuned for more info.