I'd like to see the works of modern composers like George Gershwin, Frank Ticheli, Eric Whitacre, and Mark Camphouse, along with other World composers. And yes, Shostakovich and Sibelius, while technically of the late Romantic period, would fall into the modern age of Civ, and are excellent choices.
Modern "Pop" or "Rock" music is a No. Just a No. It's not something good for just listening to in the background, and, besides, is too controversial. If they include the Beatles, what will fans of the Rolling Stones think? If they include Slipknot, how will fans of Green Day feel? If they include Hannah Montana or the Jonas Brothers, then most of their audience will play the modern age with the sound off. But I can't see anyone having much of a beef over, say, Rhapsody in Blue, Shenandoah, Godzilla Eats Las Vegas! or one of Shostakovich's Symphonies playing in the background.