With a POD no earlier than 1750, have Catholicism become seen as the ideology of the masses (in Catholic countries anyway). Perhaps the French First Republic survives and spreads anti-clericalism and republican authoritarianism across the continent, leading to the Catholic Church organising a series of revolts analogous to the Vendee and eventually becoming the primary ideological opposition to radical republicanism. Or maybe an earlier unification of the Italian Peninsular (say in 1848), or at least the permanent ending of Papal control of Rome, leads to an earlier Catholic focus on theological rather than temporal matters and a rise of liberalism within the Church, resulting in a much earlier Rerum Novarum. I look forward to reading your ideas!