Turtle Rock plans to meaningfully support Back 4 Blood post-launch, adding content that will be available for free or at a cost. The brand new roadmap released by them further breaks down their plan, giving us an insight into the upcoming content releases.
New features, such as new cards and difficulty options are on the roadmap, as well as Quality of Life improvements that aim to make the game a much more engaging experience. The Annual Pass has the meat of the content, with brand new cleaners, ridden and weapons.

I am not convinced that the Annual Pass is what this game needs to be successful long term, and might fall into the same pitfalls as their last game, Evolve by segregating their player base.
Back 4 Blood is now available for Xbox Series X|S, Xbox One, PlayStation 5, PlayStation 4 and PC. It is also available on Xbox Game Pass.