The Ecology of Dream Realms
Even dragons have to eat. No matter how fantastic the realm, its denizens are bound by rules. Even in realms where gravity is non-existent and the inhabitants regularly turn themselves inside out like gloves, there are still physical properties that govern all inhabitants. This is even true in some Nightmare Realms where the governing physical law may be that all physical laws change.
The Storyteller designing a Dream Realm needs to consider questions like: What do the inhabitants eat? If the nature of the realm is wildly different from mundane reality, for example, how does that affect seemingly unrelated elements? In the Kingdom of the Clouds, how do the inhabitants get their drinking water? Do they have water piped up from the surface, miles below, or do they have airships that ferry up the water? Perhaps they use furnaces to heat the clouds into liquid water, which they decant into containers? Maybe they don't need to worry about drinking water, as they have skin that can absorb water vapor. Is that the reason their skin is translucent, with their veins and arteries all visible on the surface?
A little extra thought about the physical nature of fantastic reality goes a long way to building a realm with a "solid" feel. All of the Dreaming's denizens are living creatures, even the ones who don't have physical form.
Plants could also play a major part in a story, beyond just backdrops. While trees and bushes are mostly used for scenery and setting the mood, a truly imaginative story could revolve around plants as either protagonists or antagonists.
Plant varieties are also important, often neglected, factor in any setting. Sure, it's simple to describe different trees as tall or bush-like, coniferous or deciduous, but the finer details are important, too. What are the differences between an oak forest and a redwood forest? Among redwoods, one feels awe at the grandeur of the trees. What kind of tree would evoke melancholy? What about humor, or frustration (the Frustrated Forest)? What are the principal shades of color one might see upon entering the Forest of Hate? Or, to turn the idea around and use color as the evoking agent, what is the primary emotion of the Crimson Wood? What lives there and why is it crimson?
Animals in the Dreaming
The Dreaming is populated by animals and creatures sprung from the mindsets of cultures and individuals. The medieval idea of animals, for example, held that each beast embodied a particular virtue or vice of human behavior. Tigers represent jealousy, elephants are symbols of chastity, eagles possess integrity, etc. Some of these animal chimera bear only superficial resemblance to their mundane "counterparts." This dichotomy is what makes the Dreaming (especially the Far and Deep Dreamings) so dangerous. Animals simply are not always what they seem here. The medieval panther chimera, or "Pard," for example, is a gentle beast that has a breath with the sweetest scent. All animals (except the dragon, which fears the breath of the Pard) in the vicinity are "charmed" by these exhalations and unable to do anything except follow the Pard wherever it may lead. (How would this charm affect pooka in animal form?) Characters traveling without a thorough understanding of the realm are in for a few nasty surprises. Dream-Craft cantrips could provide some insight, but the Art does not supply all the answers. To complicate things, chimera are changeable creatures: it's their nature. Some change when succeeding generations dream different attributes (the unicorn is a prime example: always tripping over that 20-yard horn in the Middle Ages). Some chimera are adaptable and change of their own volition.