An image of an X-Men set was posted online with the caption 'SDCC 2014' included. As you can see in the image below, which was sourced from Cool Toy Review via 4Chan, the art on the packaging of this set is based off the All-New X-Men No. 1 comic cover and features Angel, Beast, Cyclops, Marvel Girl, and Iceman figures.
This is assumed to be the 6-inch Marvel Legends line because past SDCC exclusive sets have been multi-figure sets like the X-Force set from 2012 and the Thunderbolts set in 2013. Whether or not this is all truth is still unknown and Hasbro most likely won't talk about it until Toy Fair 2014 in New York next month.