According to the latest issue of Dengeki, a third Metal Gear Online expansion is in the works, planned for release in mid-March.
Apparently named "Scene Expansion," the add-on is to include Vamp and Raiden and new characters.
You'll get three new levels as well, in Hard House, Outer Outlet and Ravaged Riverfront.
Chances of this heading west: high. We'll check now. Thanks, NeoGAF.