A completely redesigned A4 is coming in September with more efficient engines, and a diesel model may be added by the end of next year. The biggest losers in May sales were the Q7 and A8. The A8 has always been a low volume model anyway and the Q7 is getting a diesel version early next year which should help. Audi is also launching the smaller Q5 crossover next year which fits into the burgeoning mid-size segment. Audi is also working on a hybrid drivetrain, but that is still 2-3 years away from being marketed in the U.S. Audi also has the option of adding some its smaller engines that are offered in Europe to the U.S. lineup without having to radically redesign its vehicles.
[Source: Automotive News - sub. req'd]