Sovereign
Porsche Perfektionist
I think business-wise it is not the most productive use of Mercedes time. Such models need bespoke assemblies, research and development and certification. Revenue (units sold x net price) will fall way short of mass exercises such as even the CLA. And truthfully, other than us MB groupies in forums like this, I am not sure anyone is hurting for a more exclusive Mercedes sedan. Let's not forget that MB was down this road before and failed (some would say miserably) with Maybach 57 and 62.
One of the reasons I prefer Mercedes brand over its arch rivals is that Mercedes do some cars that doesn't make much business case at least to an untrained eye like mine. For example, the Sterling Moss version of the SLR had a bespoke external and interior design in the recent past and the company was ought to sell 80 cars, if I remember the numbers correctly. For such a small batch of cars they created a very different looking machine than the car on which it was based. Now the Pullman version with all its amenities is also going to be a very exclusive machine, if not design-wise. In late 1990s Mercedes created the beautiful CLK-GTR version in limited quantities and yes they also created a bespoke machine called Maybach 57/62. Today Mercedes Benz is in much stronger position financially than when they were supporting Chrysler I think. I expect from Mercedes-Benz that they would continue with their approach and create something unique, because of all the mass market producers it is this company that has the guts to look beyond the bottom line on each of their model. And if they don't create a bespoke model, it may only be that they don't want to put anything above the already prestigious S class.