It's always easy to say something is easy when you're not doing it.
If we all can agree that + time = + difficulty (considering that given enough time, you could do anything even if it's hard) then making this is pretty hard. Of course creating the models (with the right program) shouldn't be hard but it definitely requires talent. You are not creating a bunch of models for a new game, you are adding one to a game which already has models. The talent here is in creating something that fits to the current sprites and that is quite tricky.
The major problem is in implementation though. Something can be seen on @Sir Legar's post. Basically the animations are split into tiny little pieces. But into that, you need to take into account all different types of weapons and armor (which are quite a lot, for example, a spear and a trident don't look the same). Without the right tools, this process could be extremely complicated. And truth is, whichever tools blizzard used for this, we do not know.
This only accomplishes nothing in exchange of a lot of time. Imagine doing this 7 times, no. Even if the effort was done, I highly doubt 7 new sets of character sprites would go as an improvement to the ones we currently have.