I started a game playing a Romano-British duke. With invading Old Saxons and their Germanic pagan conquests, as well as the Pelagian heresy (the historical immersion is great) the moral authority of the Insular religion has dropped to crisis levels. My ruler has the ambition to strengthen the faith; while I get the reasoning behind disabling holy wars, I think it would be fun and not completely historically implausible to have something like a "pious conquest" county CB for Christians — before the Rise of Islam — that can help to fulfill the ambition (which requires holy wars, which are not available for a couple hundred years yet), strengthen moral authority and gain piety. This could work like a border dispute CB on a neighbour county and not amount to a full scale holy war.