Just wanted to express, that the heretic ruler was a former catholic.
Due to the pope and other catholic rulers around, the AAR would have more potential, then just being a heretic in the ERE, take over it and convert the Empire to iconoclast.
Let´s say, he choose to play with Toulouse, become cathar and then take over France. After that he can fight his former catholic brethren and install a catharian pope -> more potential for an AAR, but that is just my opinion.