According to a recent post on Metal Gear Online's Twitter, new DLC is planned and will include Survival Mode, new maps and more. While there has been no official date attached to this DLC or further announcements.

Survival mode isn't new to the Metal Gear Solid franchise -- there were Survival Missions in Metal Gear Solid's VR missions and a Boss Survival mode in Metal Gear Solid 2. Additionally, the original Metal Gear Online (2008) featured Survival Matches letting players compete in lobbies where win streaks equated to extra experience.