Lets play detective
"Mercedes-Benz will be expanding its engine lineup with the addition of a 3.0-liter, twin-turbo V6 powerplant with at least 335 hp and 295 lb-ft of torque.
The twin-turbo V6 powerplant will join the German automaker’s current naturally aspirated 3.5-liter V6 offering, though it’s believed that the twin-turbo engine will not only offer more performance, but better fuel efficiency. The current 3.5-liter V6 features 302 hp and 273 lb-ft of torque.
It is expected that Mercedes will outfit its E-Class models, the CLS, S-Class, ML, and GL with the new twin-turbo engine while the naturally aspirated engine will be found in the C-Class coupe and SLK roadster."
Looks like this engine with 335hp and 295 tq with a little more boost would be a perfect Z engine.
Now listen to this audi twin turbo six cylinder;
Sam's V6 3.2L Audi TT with Sports Quad Tipped Exhaust. - YouTube
sounds similar to that video engine.
Adding all the clues together it's safe to say the next G/Z engine will be a 6 turbo with ~350hp ~300tq