We've placed willow screening all around to protect the terrace from strong winds and several potted plants line the edges, but overall it lacks a theme. I like the idea of decking and a water feature, but fear the weight may be restricting. Roof gardens are magical but it can be difficult to create a coherent look when building structure is a problem. Having a uniform floor will help, and decking may be the best solution. You should have your roof checked out by an engineer to see exactly what weight it can sustain.
Wood is the lightest of any of the outdoor flooring solutions.
Perhaps you could do all your planting in the same type of containers. For instance, in the past we have planted up a roof garden using sections of galvanised metal-ducting lined with heavy plastic sheeting. Rather than filling them completely with soil we use a lot of old polystyrene from packing cases and light peat-free compost to cut down on weight. The type of plants that would grow well in this sort of situation are ones that will do well in windy, exposed situations. This could be something like the New Zealand flax, Phormium tenax , with its dramatic sword-like leaves or a selection of more modest grasses.