Posting this here because there are two notable new transformations:
1) The "shrink" debuff from Amanitar in heroic ahn'khanet persists through a port and allows the player to stay permanently tiny. This doesn't work out for dps as it reduces dmg by 75%. However, for healers, it persists until death and makes for a very difficult model to hit, and since it works in arena that can be gamebreaking. This also works with pets/minions (and doesn't reduce their dmg for some reason.).
2) The hatecoil arcanist (credit one weird guy, commenter on my youtube channel) in eye of aszhara can be pickpocketed for a massive disguise which, crucially, doesn't disappear on mount. I'd appreciate corrections but I think that's the largest mounted mob you can get.
Also, not sure how many people know this but magic mirror + skymirror disguises can enter bg's (though you can't use them inside the bg, you have to set it up before you accept the bg port) and fight, often with a very tiny and/or hard to see model, which is a lot of fun.