In terms of PoD, I was thinking a more successful Second Mexican Empire (lasting at least into the 1870s), which leads to better Franco-Austrian relations and thus a willingness of the Austrians to ally with France without the Italians.To do this, you need to make Prussia demand a more punitive peace in 1866. OTL Bismarck lobbied for a relatively light peace because he saw that Austria as a loyal ally would serve Prussia's interest more than leaving it embittered and Prussia isolated.