The idea of a sirdab is an extremely broad one, and there is not a great deal of standardization expected in the creation of any particular sirdab. As you've already read on the wiki, the only real "rule" of design is to create a space that engages as many of the senses as possible, and does so in such a way as to foster contemplation and internal harmony. For anyone else who might be curious who hasn't seen the relevant section of the wiki page, this is what it says:
"Each mosque, however large or small, has its own sirdab, a garden with a central fountain which usually lies above a cistern. These are modelled conceptually if not materially upon the Sirdab in Rahoum, and are meant to create a medley of sights, sounds, scents, and other physical sensations to draw the visitor nearer to harmony with the Song of God. A sirdab will typically include a combination of shade trees, flowering plants, scented shrubbery, fountains or pools, rocks, flagstone and gravel paths, windchimes, waterwheels, and other components calculated to soothe and captivate all the senses. These gardens are a place for contemplation and prayer, but also for physical labor, communal gatherings, and simple repose."
Both the overall design of a sirdab and its individual components vary widely. A sirdab can be fairly large, up to a few acres, or very small, only a few paces from side to side. Some sirdabs have sets of spiraling paths meant to invoke contemplation of the different harmonious spheres of existence. Others have pathways that offer different focal points and vantages along the way, encouraging meditation upon various virtues and events in the life of the Prophet, his wives, and companions. In most sirdabs the paths will center upon a stone, as in the Sirdab itself -- either a singing stone like the one there, which makes sounds in the wind; or a ringing stone, a piece of phonolite that sounds musically when struck.
Plants are an essential component of a sirdab, combining color and texture with scent and motion, even taste. Symbolic value can always be present, in plants and other garden objects, but the sense impressions produced are the largest factor in their inclusion. Roses are very common, having a scent as striking as their appearance, and fruiting trees and bushes are often incorporated too. Some sirdabs, especially those associated with mosques attached to bimaristans (hospitals), grow medicinal herbs, which are believed to have greater potency than ordinary herbs. Typically noxious plants would not be present, but if a plant has both beneficial and harmful aspects, whether its a medicinal plant or a thorn-studded rose, these may be included not only for their usefulness or beauty but because they foster reflection on balance and the nature of existence.
The people of the caliphate love water -- the sound of it, the smell, the reflections and the play of light and wind -- so most sirdabs do incorporate water in some way. When possible an underground cistern will help feed such features and water the plantings as well. Where funds and space are limited, a water feature can be as simple as a brass pan of water that produces ripples in a breeze. Both water and vegetation additionally encourage the presence of wildlife, especially birds. Birds are considered a vital part of the sirdab, filling the air with their song and ornamenting it with their plumage. The sirdab at the provincial court in Omrazir has an aviary! Frogs, turtles, fish, bees, and butterflies can be other living accents of a sirdab. Not all ornamentation need be natural either. The Sirdabi in particular have a great love for clockwork and are inveterate tinkerers, so some sirdabs can have clockwork birds and creatures that move and make noises. Some feature moving models of the heavens.
Someone designing a new sirdab would have great flexibility in doing so. The most important thing to consider is how the layout and makeup of the sirdab will foster harmony with the Song, and help people attune themselves to it and meditate upon its nature and the still deeper mystery of God. There is actually a professional class of people called Cultivators who make it part of their life's work to design sirdabs! They consider it their calling to cultivate not just plantings and landscapes, but the faithful's relationship to the Song. Such work combines a certain mysticism with practical skill and hands-on labor. Not all sirdabs are designed by one of these people, but often they're at least consulted -- most Cultivators would consider it wrong to charge a fee for the work and will give their labor where needed, though they accept whatever people are willing to give them, including lodging and meals for the duration of their work.
As for Omrazir, yes, there are many sirdabs in Omrazir because there are many mosques! Some of them will be built into the game and able to be visited when the story eventually moves to Omrazir. Others could be designed for individual neighborhoods by players themselves.
To make a very long-winded story short -- there is vast scope for player creativity in imagining a sirdab!
