As the topic says. Personally I think Magus Sisters could be cool, or maybe a fun gimmicky battle like Ragnarok from FFVI. I can imagine it being a giant floating shapeshifting sword with one of the mechanics being that it can turn random players into some sort of object that needs to be killed to change them back, something similar to the Calca/Brina dolls in the Antitower. I grant you the latter probably isn't likely, but who knows?