It would be nice to have a layer that's just color and opacity for the whole map. If you want a battle at dusk make it a nice golden yellow or pumpkin color. Want it to be misty? Set it to white and play with the opacity. Sort of like adding a custom filter. Perhaps you could even add non-animated particle effects that's just a still image repeated? Currently you could color adjust all your maps in photoshop and upload them but all of the tokens would have to be color matched or you'll have to play around with the overlay colors provided to not make them stick out like a sore thumb on the battlefield. It seems like it'd be simple maybe its part of the lighting layer? Perhaps even an option that colored lights override this layer so you can have everything set to sort of a purplish grey for nighttime but a character with a torch has a yellowish light around them. Maybe its as simple as allowing lights to be in the foreground instead? I don't know, it seems like it could be a pretty easy addition but who knows. It just adds a little extra depth.