German luxury-car maker Audi AG (NASDAQOTH: AUDVF) threw down a big gauntlet this past week: It showed a photo of an upcoming electric luxury sedan that is very clearly aimed at Tesla's (NASDAQ: TSLA) groundbreaking Model S. The Audi e-tron GT is likely to be the third Tesla-like vehicle from the German luxury brand, following an SUV and a sporty crossover.