SonyAlphaRumors (via Asobinet) points out that Sigma has filed a couple patents for super telephoto lenses; a 300mm f/2.8 and a 400mm f/4. The designs appear to be rather forward-heavy, with a great portion of the glass far from the camera mount. That might feel like wielding a sledgehammer to some.
The lenses appear to be designed for the Sony E-mount system, featuring the retrofocal design elements needed to fit the image through the relatively skinny E-mount. L-mount users could expect versions to be produced for them as well.
While some companies make many patents relative to each lens actually produced, Sigma’s pattern has been to patent the ones it intends to put into production. Its last foray into super telephoto lenses was the quite good 500mm F/4, which was just about as good as the Canon equivalent of the day, although was not as good at taking teleconverters.