No, this does not mean that Messenger will become its own OS, but rather Facebook is said to be trying to emulate the style of messenger apps such as WeChat and LINE, both of which offer more than just messaging, such as separate apps for photo editing, wallpapers, games, and more. Snapchat is also another example of how a messaging app is starting to integrate other features, such as video content into it.
Facebook is also said to be looking into integrating information from third-parties into Messenger. The venture is said to be slow at the start with Facebook working with preferred partners, but it is expected to pick up speed as Facebook eventually opens its doors to more developers. Of course this is pending the initial success of the platform.
In fact we have sort of seen Facebook begin to branch out with Messenger with the release of the Stickered for Messenger app that allows users to add stickers to photos. Facebook is said to share more information about their plans for Messenger at their F8 Conference which will run from the 25-26th of March.