The ground has already been covered and click this for an excellent coverage of the 25(!) varieties currently available. At last count, anyway.
It used to axiomatic in the marketing world that you didn't mess with success. And, you most certainly didn't line extend your biggest, most profitable product. The reasoning being that any variations from the tried and true original would only cannibalize the market share of the original. In other words, same market share, less profit. So we're way past that simplistic approach. Listerine, for example, now has how many versions? In my time the word at Warner Lambert was "You didn't mess with Big Yellow." Same in my time working with the makers of Twinkies. No variations! Times have changed.
Anyhow, here is my fantasy of an extreme version of Oreo's that might be coming to a shelf near you only too soon.
And, if it doesn't, here is my idea for a competing brand that will fill the niche for that ultra-stuffed sandwich cookie.