Problem: If you create a new style or a symbol, it is only available for the open document. This isn't convenient as you need to either use one design with multiple pages acting as different designs, or copy and paste between designs.
At a basic level, this could be solved if we were able to set styles and symbols as "global" so multiple design files can be created for a single client without having to copy any styles/symbols across from an existing document.
An even better solution would allow the ability to save named sets of styles/symbols and the ability to import these into your design. Then you could create a set for each client and import to any new designs they want, without being cluttered with any other clients styles/symbols.