According to leaked artwork by YouTuber "TheFamilyVideoGamers", the next Call of Duty title will be called "Call of Duty: WWII", and, as the name suggests, be set in World War 2. The main artwork depicts a squad of soldiers on a beach under fire, presumably during an invasion similar to D-Day.
This fits in line with Activision's earlier claim that CoD 2017 would "take Call of Duty back to its roots."
To prove his authenticity, the user also claims to have leaked Infinite Warfare in the past, under the alias of "InfiniteWarfareLeaks". Apparently, this account was removed by Activision after they filed a DMCA complaint. The original post can be found here.
Charlie Intel has dug up some old quotes from SHG/Activision staff to support the leak.
In 2014, Michael Condrey stated his willingness to see another CoD game set in World War 2: "I think a next generation game with the latest production values and robustness in a World War II setting like Band of Brothers would be amazing."
In 2015, Activision Publishing CEO Eric Hirshberg discussed in an interview the validity of the setting, stating it was “certainly possible” it would be used in a future game: “Those kinds of things happen,” Hirshberg said, “so there’s definitely a chasing of innovation and new ideas within a well-worn genre that lead people to similar conclusions like changing time periods. And of course I think that opens up the opportunity for what is old is new again.”
Anyway, since this seems fitting, here's a poll: