No, it wasn't. It was general concept model, most likely specifically made for him.
After they released the final product, Bashiok was allowed to change his avatar to the final version.
Besides, it's not like anyone thought the Fallen wouldn't be coming back.
Pretty much everything we have seen is a general concept model....as it is all subject to change....atleast a
little......And yes it was fairly safe to assume that the fallen would return.....however it was not safe to assume that bash's avatar was the fallen as it is still not safe to assume what the caldeum chars are.... i will agree it is unlikely they would hide a goody like this right infront of us...but blizz has never been short on surprises