Bleeding Cool has been talking up Avengers #675 for a couple of weeks or so, noting that it was missing from released solicitations last week, but expected to be heavily in Marvel Comics' printed solicits this week. And how certain retailers have already been tipped off that it will represent something big for 2018.
It appears that the issue in question will begin a new weekly Avengers story, entitles No Surrender, as leaked by ComicBook. It suggests one story across Marvel's various Avengers titles, once a week, or even that each team will create an Avengers issue, each month, turning the main book weekly.
Avengers #675 will launch on January 10th with a big "party" promotion and many variant covers. No Surrender will see creators from across the Avengers line participate, including Mark Waid, Al Ewing, Jim Zub, Pepe Larraz, Kim Jacinto and Paco Medina. This may be the big event maxi-series involving Larraz we have been told to expect or that may still be something else.