I didn't think Springfield ever said what the "S" stands for! Some say Slim, others say Subcompact, while others claim Single Stack.
Personally, I would like to see a 7 or 8 round 5" model. I really like my XDs .45 as an everyday carry pistol. I also like the longer sight radius of my 5.25" .45. But the double stack makes it a little too bulky for my frame to carry in the summer months.
As a side note, I refuse to carry my XDs with the 7 round mag, as it hangs on the heel of my hand when reloading. I know, I know, all I have to do is "rip the mag", rather than "drop the mag"...but if bad things happen, I don't want to think about my mag changes.