Spoiler:

Ok guys BIG surprise! I will be unveiling Sarena-San's curse form the Great HiHi Transformation later, but here's my reason. Goku has Super Saiyan God state and I realized her form contradicted his be making her pure evil and unpredictable so much so that something awful happens once she achieves this form.

Some probably are going to debunk my idea for this but I PROMISE it will make sense, oh and don't expect it to be similar to super saiyan forms because its not genetic to Saiyans-it was a hidden plague that will finally break free!