The word "Diet" and the existence of co-voivodes led me to assuming it was elective. I'll stick with it.Nice update.
I'm not sure if the Voidvodship of Transylvania was back then elective... The Voivode of Transylvania was the highest dignity in the Kingdom of Hungary and was usually appointed by the King, as fair I know up to the conquest of Hungary by the Ottomans when it become independent (well, not independent but subject of the Porte). I might be wrong. The Korvins initial power-base was Transylvania and they remained vastly popular there.
The Korvins are indeed popular, but they also represent royal power, and thus Jànos is meeting opposition by principle.
IOTL Stephen IV Bathory was cruel especially to Szekelys. Give him ten more years, and you can only get more of that.For Janos to become the Voivode of Transylvania will be un-usually. He could appoint his son (not sure if he has one), a good friend, a relative or someone who trusted. The province was the crown jewel, very important in military power, having an army of its own quite powerful (at least 20.000 men, even more in terms of danger) and a decent economy. But the most important, it was the source of minerals: gold, silver, iron, salt, etc. It was very important to have it very well administrated and defended.
Concerning the status of Szekelys, they were part of the Unio Trium Nationum (Szekelys, Maghiars and Germans) established in 1438, which were privileged nations and worked together to resist the king and to keep the peasants down (and the heretics, aka Vlachs). The Szekelys held even more power as they were the military branch of this agreement. While the Magyars nobility have troops of their own, the Szekelys were organised in military "Seats" with their own commanders, weapons and were exempted by most of taxes. Some of the Szekelys's privileges were indeed suppressed by the Bathory's voivodes later, and make them swear enemy of the voivode (for ex. thousands of Szekelys served in Michael the Brave army against Batory in 1600).
So, in this TL, they loose their privileges earlier, when the voivode was opposed to the king... not wise for the voivode... not wise at all! suicidal I would say.
Ottoman succession is still the shitshow it usually is, but for now the Sultan has in Korkut a ready heir he will probably assassinate as soon as he has a son. Selimsah is also available, unless he's been killed by rebellious Greeks.Appointing Stephen the Great as Voivode of Transylvania is a little far fetched. He held many lands, some 40 villages and several fortresses in Transylvania, however he was schismatic and a Vlach and a foreigner ruler. I could see one of his sons, even Bogdan, to be offered the title until his father die and he take his place in Moldavia, but renounce to Transylvania.
Both Moldavia and Wallachia were a kind of Hungarian vassals. Theirs voivodes sometime played the good vassals, other time the good allies other time the cursed enemies. Stephen was the master in playing the art of diplomacy (and the war), pitting the others powers against each-other. When he was enemy of Hungary, he was friend of Poland and vassal of the Turks. When he was enemy of the Turks, he was allied with Hungary and Poland. When he was enemy of Hungary he was... you got the point.
So, were the Ottomans fixed their succession? How fare the Persians and the Mamelukes?
Good idea for Bogdan instead of Stephan Musat. However, Jànos's son would be a mite too young to be Voivode, unless the King serves as regent for Transylvania...