Egami (translation here) spotted a new Tamron patent describing a 300mm f/2.8 lens for a Full Frame mirrorless camera system. As you know Sony Corporation maintains an 11% share hold in Tamron (Source: Wikipedia). So this lens may be designed for FF NEX cameras…
P.S.: Tokina just admitted that The Kenko 400mm f/8.0 lens for Sony E-mount has a defect on the shutter. You should contact Tokina to exchange the lens (Source: DC.watch).