A new teaser has been released by Marvel Comics promoting a "Death of X." Listed for release this fall 2016, no other details were given.
Of course, the most logical association for the "X" would be Marvel's X-Men. Several major X-Men characters are already dead, and the surviving mutants are dealing with the fatal Terrigen Mists permeating Earth as well as a new level of persecution from the still-unspecified actions of the now-dead Cyclops during the eight-month gap between Secret Wars and "All-New All-Different Marvel."
Of course, some Marvel consirpacy theorists could see this as a precursor to ultimately killing off their mutant-verse (which 20th Century Fox holds the media rights to) in favor of their Inhuman corner of the universe which Marvel Studios does control.
Another possibility is the long-dormant Earth X. That alternate universe series had a similiar red "X" in the logo, and was touched upon briefly in last year's Secret Wars event.
Finally, "X" could be just a placeholder referring to a character to be named at a later date.
Look for more details about Marvel's "Death of X" here on Newsarama as it's revealed.