Bright vs contrasty, raw vs finished, grid layouts vs organic blobs....seems that swinging between opposites is important for variety, yet keeping a thread of consistency is what creates the magic of a believable world. I spend a lot of time thinking about this. (Anyone else??) Meantime, there's no substitute for just doing it daily and seeing what happens. Maybe walking away from images for a day or a week to see if I like it better, later.
To give credit to some of my childhood influences, who did manage the balance beam of exploring variety while keeping that recognizeable touch, two favorites off the top of my head: the Dillons (Leo & Diane, jointly created all their artwork together) and Lisbeth Zwerger (otherworldly watercolors, she adapted many fairytale classics). I tried to find every book of theirs in the library and was elated whenever I crossed paths with a new one I hadn't yet seen.
If you have time to peruse some images:
Zwerger's Swan Lake here
the Dillons— nice collection of imagery here