Why No 21" Rims Offered as an Option?
 

Sometimes I look at my wheels and they even seem small sometimes in the wheel well depending on the angle.

If MB offered a AMG 21" wheel I would have gotten it.

I live in CA and a "stock" 21" rim would look nice.

The wheel wells are not big enough for this size wheel unless you wrap the wheel in a rubber band. Too much damage potential to the rim and suspension as well as jacking up the ride which is what most people are buying this car for.

Even if you could put 21" rims and tires on the car, which wouldn't fit well at all, the quality of the ride would be horrible. Most people buy a W222 for the ride quality, as in the almost total lack of any road feedback. What you want to do would create a very harsh and uncomfortable ride for most W222 owners.

You may not be able to tell but this is simple physics here and you can't avoid the laws of physics.

I don't see how you can fill the wheel wells more by getting a larger size rim. When you get a larger size Mercedes option rim the tire size diameter does not change. That way the engine rpm's and the suspension geometry does not change. (How it may affect the semi-autonomous sensing/action is questionable)
Of course you can go after market and do what you think will fit.
Most of the tire dealers in Canada won't even put non OEM recommended sizes due to safety.
Bigger rims might make it appear like the wheel well is fuller.

If you want looks--you can put whatever wheels you like on the car.
In my opinion if you do not increase the size of the rotors, it looks Ghetto.
Endurance cars use extreme wheels to put bigger brakes -- not for stopping power, but for longevity.

Best ride is at 18 or 19 inch wheels.
Best traction is at 18 or 19 inch wheels.