Hi there,
I think it's a great idea to have a theme running through a garden!, to give you a peacefull and tranquil to relax and unwind.
My advice would be to, firstly concentrate on a focal point, remembering the four elements, Earth, wind, fire and water, the last one, you may wish to introduce a water feature, water trickleling over stone can be very effective.
Think carefully where you place your focal point, it may take several moves before your happy with it and once you've made your final choice, I think everything will fall into place as it gradually evolves. Seating may be something you want to think about too ie will you want to catch the moring sun or afternoon sun.
Read up on mythology, be it Celtic, Norse, roman, Eastern, North/south American etc, and go with one that appeals to you most strongly, Japanese gardens are very popular and reasonably easy to maintain, I find bamboo wind chimes very evocative and more gentle than some of the metal ones. Archways or pergolas also add interest and can devide the garden into little hidden, secret compartment that invite folks to explore. Scented flowers or foliage will also add charm and characture, theres nothing like the scent of lavender as you brush past it. Good Luck Tbird+