when I saw the "D" tweet twenty minutes ago, I also first thought "Dual" motor. I agree it would be Model SD or something like that rather than a Model D.
I think "D" as dual motor makes sense as it will emphasize that this in not simply Tesla's 4WD, but rather an entire set of improvements that come with having two motors in a Tesla EV... something consumers will not find from a gasoline car.
I hope that's what this is. If the "D" is the driver assist, I'll be a bit bummed, only because the much of the cat is already outside of the bag on that one.
Personally, I really hope the "something else" is a bigger battery as someone suggested upstream. Managing my expectations, I'm thinking it will be a formal reveal of the driver assist package Model S's have started to be build with hardware for. If it's the X reveal, well, Awesomeness