Judging by these photos of a prototype, the changes will include subtle tweaks to the front and rear fascias as well as new internals for the lights at both ends. We can also see that the front grille is wider and lower than on the current S5.
Look for the same tweaks to appear on updated versions of the S5 Sportback and S5 Cabriolet. Similar changes should also filter across to the regular A5 and high-performance RS 5.
There's no shot of the cabin but inside, the dashboard could be redesigned so that the infotainment screen is integrated within the center stack instead of tacked on to the top of the dash like in the current S5. We might also see the controls for the climate settings switch from physical knobs to a touchscreen interface like in the latest A6, though physical controls could remain if Audi goes with a design similar to the one used in the A1.
2018 Audi S5
Power in the current S5 comes from a 3.0-liter turbocharged V-6 good for 354 horsepower and 369 pound-feet of torque, with the engine mated to an 8-speed automatic transmission and all-wheel drive as standard.
The same mechanical package should feature in the updated S5, though we could see a 48-volt mild-hybrid system added. This is essentially a belt-driven motor-generator that can recover energy under braking or coasting and store this as electricity in a small battery located in the trunk. The energy can then be used for starting the engine and helping out during high-load situations.
The mild-hybrid system has been added to the latest S6 and S7 where it is also used to power an electric compressor that helps build boost pressure before the engine builds up revs and turbocharging takes over.
The current A5 range was introduced for 2018 so we will likely see the updated one introduced for 2021. A debut should happen in the first half of 2020.