What if the future Alexander III of Russia renounces his rights to the Russian throne in 1866 so that he could marry the then-love of his life--Marie Mershensky (sp?)? Indeed, he wanted to do this in our TL but his father prevented him from doing this; thus, let's have Russian Tsar Alexander II agree to this in this TL.
Anyway, what kind of Tsar would Alexander III's younger brother Vladimir be?