My main issue is not that FGs will become too easy, but that people would effectively reduce the amount of FGs they fight against. People will simply play realm that gives them best win percentage all the time.
And it would be impossible for Zanz to rebalance gods all the time to make them, and keep them, at same difficulty level.
However, if they are randomly shuffled every week, you dont have that issue. Instead you'll have easier and harder weeks, but at least you wont render 18 gods useless.