Firstly, cater to the appropriate market again. I feel like M-B is starting to design their cars to fill a role of the Fast & Furious crowd within the luxury segment, if that could even make sense. Short sighted designs and strategies which are looking to be less of a premium manufacturer and more like a mass-market one.
Be a design leader again, and by that I don't mean scribble nonsensical lines to try and look ultra-fad which will generate quick attention then date quickly. I want to see a monolithic, powerful, confident and GERMAN looking Benz again. Strong lines, connecting dots, an unmistakable presence, a clean and concise surface that sends a strong message of strength, simple beauty, harmony, and timelessness, and of course prestige. M-B is designing cars to look broken up, therefore M-B's today look stubbier and shorter than their competing counterparts. Audi's and BMW's with one long graceful line look elegant and long. M-B is trying to be "crazy different" but it's not having an elegant and "sprawled out" effect we like to see from long cars.
IMO be like Apple in the sense where if a product isn't working, cut it as quickly as possible, to not dilute and harm the brands ultimate cachet. Dilution is a dangerous thing for a high margin luxury brand when a lot of their models can't even be considered top-tier within their segments, not to mention when you have scars of future embarrassments like the R Class, the first A-Class, etc.
As an E-Class owner, I've gotten in plenty of arguments with 5-Series owners who say that the E-Class does NOTHING better. These days, although I still love my car, it's hard to find objective areas to prove otherwise.
Areas where I'd like to see improved: INTERIOR MATERIAL QUALITY. Aside from the S-Class/CL/and new SL, I think M-B models interiors aren't so refined and detailed as they used to be. Cold plastics, austere feelings, general "cheap" materials here and there. Though, they do feel durable as all hell, they just don't feel "warm".
I think M-B needs to figure out what they want to be in terms of driving style and go hard with it. They used to be the NO NONSENSE luxury driver. Now, they're trying to get a little "BMW handling" on us (BMW is going the other way, ironically, generating the same complaints but on the opposite end). I find my E-Class to be a confused ride. Soft on corners and smooth roads (so it floats), then it gets hard and extremely harsh over bumps. It does the wrong things at the wrong times. It should float and be soft over harsh roads, and get stiff and tight through turns, right now it does the opposite.
I think the engines are fine, personally. But the 7G Tronic transmission is a joke. My 2011 E now has the same "hiccup" that my W211 did, which had to go to the shop about 5 times before they finally changed the valve body. When it works well, it works well, but there are lots of inherent bugs. It could also be A LOT snappier on shifting, without sacrificing the smoothness.
The screen sizes don't bug me, but they could get bigger.
Fix the suspension creak/squeak/groans. Every M-B I've had has this characteristic and "old man car" sounding sound. Even my 2 brand new E's. I'm skinny as heck yet the car creaks/squeaks when I get in or out of it. Embarrassing.
TAKE THE HORN OFF OF THE ALARM! THIS STUPID COST CUTTING MEASURE MAKES OUR CARS SOUND LIKE CHEAP CHEVY RENTALS, WE SHOULDN'T GET SUCH A CRUDE GREETING WHEN WE SPEND PREMIUM DOLLARS.
Fortunately, for M-B, they have over 100 years of prestige to allow them tons of apologists who will look past current shortcomings, or even convince themselves that M-B is releasing their best products ever. I was one of these people, but I couldn't hold onto the lie.
Finally, I feel like participating in BMW message boards, and Porsche message boards is far more fun and enlightening. BMW enthusiasts seem to be far more product-driven, far more enthusiastic, they're even far more demanding and far more criticizing. I find M-B boards can be a bit clinical, i.e people just want the Star Car and aren't too crazy passionate about it to adore it endlessly or criticize it too much, just kind of chillaxed as compared to BMW and Porsche fans. That can be seen as a good or bad, but from my experiences, considering I'm obviously one of those crazy anal fans, I'd like to see M-B make products that attract the "Intellectual Enthusiast", rather than the "I just want a Benz" crowd. On the M-B boards, 90% of the Threads are about "rims" or lowering or changing out for a cheesy Star grille or something. On the BMW Boards, you have real deal, introspective enthusiast talks, some of it goes far more fanboy-ish than anything I've seen, but the passion in the core product is there.