Backyard Imagineering isn't anything new, it is what Imagineering is. Wherever Walt Disney was when he needed to create, it was primarily behind his main establishment that the ideas took shape. As a boy: the barn out back, where he drew pictures with tar on the side while his parents were away. As a young man, he used the garage at his Uncle's home in Hollywood to create his first animation out West. As a successful man, he joined the industry who created the cineramic dreams on the back lot. When Walt thought he'd have something to keep guests at his studio entertained, he planned a theme park behind his studio. Later, when his plans became too large for that property, he created Disneyland, the greatest amusement park on earth in the backyard of his residence there on Main Street. So you see, if you have a backyard and a little imagination, you're right in there with the best of them.
Every backyard has it's own personality. There isn't any reason why you cannot create the kind of motif your heart has always been looking for. Don't let your backyard's present state dictate what you can do if it's limitations are due to what you installed from the local hardware store. There is much more you can create, and if you break the norm you get what you always wanted anyway. Your thoughts are distinct and original, and so should your home be. Some may say a themed backyard is eccentric. The way I see it, it would be eccentric to have someone else's yard rather than your own. The personality of your backyard can be hidden behind the dictates of the norm.
The first thing you should do is make a list of what things go together with what you'd like to go for: A waterfall, palm frond roofing, seashells, netting, torches; miniature millstream, lanterns, shack, horseshoes, campfire area, etc. There are treatments for all flammable materials these days, so you're not as limited. I'm not just saying this, but I can't think of a motif that fireflies do not go with.
Walt Disney's approach in doing this was to consult his complete collection of National Geographic Magazines for authentic pictoral ideas. There are various sources you can use on the Internet.
You don't have to live in someone else's yard, but rather in your own dream.