If you want contrast then you want hard light striking across your subject/scene, much like the sun. Contrast is caused because you get highlights and shadows. If the shadows vanish the contrast drops. If the highlights vanish too then you will get flat images.
Adding on camera flash won't really help much. It will fill in what might have been shadowy holes and furrows in the textres of the scene, which is kind of counter-productive. Consider a full moon. That a bit like lighting the scene from the camera position, as though on camera. You can't make out texture in the surface at all. It is evenly lit and rather a dull image. That's with hard light - a small light source relative to the distances involved and very directional. Now think of a partially lit moon, with the sun lighting it from one side. Suddenly the texture of the craters appears as shadows form in and behind craters and highlights reflect off the edges.
The bit that has the most visible contrast and texture is the part where the sun is glancing across the surface relative to the camera position. The fully lit, shadowless part has barely discernable features.
When you have dull, grey, cloudy days the light is coming from all around. It fills in every nook and cranny. There are no shadows. There are no specular highlights. It is the softest natural light imaginable - the cloud cover is a giant softbox.
So, if you want to add punch then add hard light from the side, i.e. off camera flash. However, because of the inverse square law and the rate of light falloff with increasing distance you'll have a problem lighting a large scene like that, and even more of a problem lighting it evenly. One little flash gun doesn't quite have the power of that big ball of fire above us.
I think you've done a good job at pepping the scene up with your edit, but I think that if you want to avoid doing that you will need nature to provide the right sort of light. That means choosing your day, time, shooting direction and weather carefully. If the light is not right you won't get the shot you want.
