Quote from thehuw »The Kirin cycle (from Saviors of Kamigawa) might be too powerful.
Any four-mana spell with transmute is Diabolic Tutor for three, which seems a bit much. Fleshwrither is similarly powerful.
Droning Bureaucrats can lock down everything if you have the mana.
Hisoka, Minamo Sensei is basically unlimited 2U counterspells in this format.
These are the potential problem cards I can think of, off the top of my head, but this is a casual format I might be able to see myself playing.