Leave it empty.
At ground level a sign reads: “Look up and let your mind run rampant. What do you imagine is, or could be, there? Stand back, think and create it in your mind.”
The sign then points you in the direction of an app and a website where you can describe in writing (or drawings, if you find it easier) your idea for a Fourth Plinth sculpture. These ideas are then presented in a notepad gallery within the app and on the website.
In a nearby building, possibly The National Gallery, tablets are placed close to windows overlooking the fourth plinth, allowing people to create their ideas closeby on a larger screen than their smartphone. If it can be done securely enough, posts could be set up at different points around Trafalgar Square with built-in tablets for the same purpose; and allowing different vantage points to imagine from.
Advertising in newspapers, on social media and transport would alert people to the app and website, encouraging people all over the world to take part.
Although there is always the option to keep your idea as imaginary art only.
