Reading the excellent 1970s architecture book "Dallasights," it turns out that the Dallas Market Center's World Trade Center began as a seven-storey building with full-height atrium, with foundations underneath for a twenty-storey building. As the skyscraper sites will all tell us, the World Trade Center is 15 storeys after an eight-storey second phase was added above the first phase.
Now this means, most likely, that significant additional levels could still be added -- but the demand isn't for the original use, so what would YOU add up top?
For whom, and why for that blend of people?
Your design can also extend to the ground on one or both long blank walls of the existing building, if you like.