Add additional zones. If the controller and wiring is an issue, sell the customer on the Nelson wireless valves that are battery operated. They are a snap to install, and can give you unlimited opportunities of additional irrigation (as long as you extend your main line).
With the battery operated, there is no link between the existing controller and wiring. It operates solo (per valve of course). Make sure you don't exceed your 12 GPM (if on residential 3/4 meter)!!!