Great potential in this scene! I think I definitely feel what you are trying to achieve here, scenes like these are on my TG bucketlist too, so to say
Given your experience and TG expertise I think you can definitely pull off to make this more photorealistic.
At the moment I find the indirect lighting very bright and too green. Overall contrast is too low, especially if you consider how a photograph would look when made under similar lighting conditions in the real world.
As Hannes said it looks flat because of these two issues. The path tracer is vastly superior to the legacy renderer when it comes to shadow depth and indirect lighting accuracy, so this scene should definitely be a very suitable one for it.