Back in December 2018, FromSoftware president Hidetaka Miyazaki said the company had two unannounced titles in development, one of which was later announced as Elden Ring. Prior to that, in June 2018, Miyazaki hinted that a new Armored Core was in development, asking fans to “please wait a little while longer.”
Thanks, Games Talk.
Update 12/04/19 at 7:50 a.m.: Game Spark got in touch with Yasunori Ogura and asked about the image, who confirmed it is not related to a new title. He simply has not set a new header image since the launch of Metal Wolf Chaos XD in August, and had no intention of suggesting a new title. The image itself is from Amored Core: Verdict Day. It was not used much, so it was misunderstood as a new image.