I don't think you'll see any more diesel cars in US until the ultra-low sufur fuel standard goes in effect next summer. It's simply too hard to build an efficient catalytic converter that can tolerate high-sulfur fuel. That said, it's just a matter of time before more diesel cars are sold in US. Fuel economy used to be a non-issue in US, but that's changing. Diesel engines used to put out too much NOx to pass CA and NY emissions tests, but that's changing too.

It's really cool that someone actually sends requests to manufacturers asking for these things. I work for a large corporation myself, and in my experience direct customer input like this is surprisingly effective. One would think that a company like MB already pumps lots of money into market research, and knows all there's to know, and a few customers asking for things would have zero impact. The reality is often more complicated. I personally believe that most "market research" is total BS, the results that marketing people come up with are simply what their boss wanted to see, for whatever internal political reasons. This is how you get things like GM saying "Americans don't want fuel-efficient cars, we have done market research so we know". Then of course Toyota proves them to be total idiots. So direct requests sent to sales offices (this is a different branch of company than marketing, which is important) do tend to have appreciable impact.

Unfortunately, no sales rep from any half-way-big company would tell you anything specific about upcoming product releases. The corporate mentality is to keep those things as secret as possible; you'd have to sign an NDA to find out anything at all about future plans.