I'm not sure if this is the right place to post this, or I should take this to the Misty Mountains Faction Outline, but I'm sure a mod will move it if it's wrong.
I must say I was rather disappointed reading the MM Faction Outline and finding no mention of the Stone Giants. Some people might find them silly, but to me they were an iconic part of the original goblins faction without being too strange or out of canon like Gorkil or his scorpion. Their absence saddens me.
The MM faction doesn't seem to possess a dedicated siege unit, other than possibly the Wyrms. I love the Wyrms, and while I could easily them burning down the wooden structures of Rohan, they'd be less suited against buildings made of stone. Not to mention, while it may be part of the MM's faction design, they're very close ranged and don't really fill the niche of a catapult like a boulder-throwing giant would. And more beasties are always a good thing when you're an Orc of the Misty Mountains.
Now of course, the AOTR team has already proven the quality of their work time and time again. They might feel the giants from the original BFME II don't meet their standards. And that's okay. If they feel that way, I completely understand, but I have a further suggestion. It might mean some extra work but I imagine they're up to the challenge.
Give them the redesign from the Hobbit movie:
Some would argue that Peter Jackson's visualization doesn't fit the vision of Tolkien, but the AOTR team has already displayed they're more willing to follow the movie's ideas than the Edain team(not trying to bash the Edain devs), otherwise Bolg would be the MM's faction leader instead of Azog. (Also, when I first read the Hobbit before the movies came out, I actually pictured the Stone Giants as being literally made of stone, though maybe I'm just an idiot.)
A new model and texture would be necessary, but could be applied to the animations of the original unit. This would of course call for the giants to be significantly smaller than they were in the movie, but several units have already had size adjustments so that's not much of a problem.
As an added bonus, this black-stone monster would not look out of place emerging from the similarly colored Lair of Wyrms, giving a structure previously dedicated to only a single unit a dual purpose as the MM's siege works. The giants would form a sort of duality with the Wyrms, the Wyrms being close-ranged unit slayers and the giants providing long ranged artillery support.