Listen man, I don't think anybody is going to come out with a mod/plugin that makes you into a group of animals, or an animal. Nor will shape shifting ever be possible in the RP universe, let alone viable, with the exception of Evergrowth magic if you are a Yanar, and Soul Magic as stated above. Also, changing the hitbox of yourself is cheating. If anybody who is reading this saw my feedback on the Diaboli thread in player lore, you can tell where this could be going. I could make a long, bullet list on how this is completely unfair, and is not compliant at all with the lore, with the few exceptions above.
To sum up my critiques: The PvP unfairness would be a big repellent to massivecraft PvPers, which makes up, what, half of the server? Even though it wouldn't be given out to any person in particular, and would require an in detail application (Since you say long, it doesn't really need to be if it is to the point and gives detail,) staff can't truly trust anybody outside of their circle.
As a big lore critic, I can say that there is no possible way for this to work RP wise. Again, with the couple of exceptions above. There is no werewolf curse, nor will there be. There was a long and in depth discussion about this in the General chat which ended in a lore staff (I can't remember which,) had to end it, and change the subject, confirming that no werewolf curse will ever be set adrift.
You say that the people who would have this ability could turn into one animal/mob. Most mobs in minecraft aren't lore compliant (i.e. Enderman, Creeper, Silverfish, Slime, e.t.c.) Most of the animals are, but again, there isn't a way for this to happen without the use of Soul magic.
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Again, I could go on, but I don't want to turn this into a book critique. You get my point. There are just too many holes in this plan, and said holes are too big to be patched up.