Just add one type of prayer and rework the spammable praying system to something similar to rune magic. That would make it less spammable and also nerf op and broken volkmar. The additional prayer and rework of certain prayers could counterbalance any possible nerf at least somewhat.
Don't u love it when volkmar spams all his prayers all at once? I don't think sigmar or whoever it is would be so keen to grant him all his prayers all the time