According to Fujirumors, Sigma will announce an X-mount version of the 100-400mm F5-6.3 DG DN OS Contemporary lens.
This lens wil be on sale later this year. Adapting full-frame lenses for APS-C is not a new approach, like the Tamron 150-500mm F5-6.7, for example.
The X mount already has a genuine XF 100-400mm F4.5-5.6 lens, but it costs $1,899. This Sigma lens for X mount will be very popular if it is released.