>>18143838 (OP)
I'm not picking Byzantines because they have had their chance and wasted it.
I pick the Tsardom of Bulgaria, invade Thracia (I don't have the cannons to take Constantinople yet) and seize Gallipoli, then I constantly keep an eye on the Turks as I expand in the Western Balkans and/or Greece.
I'm already a Caesar formally, so eventually I can simply proclaim myself the new Roman emperor.
I'll make lots of concessions to the Italians and the Knights Hospitalier, better to let go of some cities than lose all the land to the Ottomans