The title says it all: how about a centerline-thrust twin? The Cessna 337 (military: O-2) Skymaster would be a lot of fun. We've already seen twin-boom aircraft in the P-38, the FT Bronco (which looks to be based upon the OV-10, which replaced the Skymaster), and the Sea Duck, so why not try a centerline-thrust twin? I think it'd be pretty neat.