As previously stated, Sigma is part of the L-mount initiative. Not only as a lens vendor. In a press release, Sigma state they’ll develop an L-mount camera:
All future interchangeable-lens camera systems developed by SIGMA will feature the L-mount. SIGMA does not plan to develop new cameras equipped with an SA-mount.
Whether they’ll use the Foveon sensor technology or not is a question that is not answered there. I think they make the right choice here to not create their own mount.