I think it's Hungary. Suleiman the Magnificent never wanted to conquer Hungary. He se the Danube as frontier of his euopean empire & wanted to make it a stable buffer state. Wedged between the Ottomans and the Habsburgs. But Louis II decided to be buddy-buddy with Ferdinand of Austria (how could he not? His wife is Ferdinand's sister) so Suleiman wanted to teach him a lesson.
It went too far....at Mohacs Louis died along with the cream of Hungarian nobility. Under some kind of prenup agreement, Hungary was to be joined with Austria if Louis died without a child. So Ferdinand laid claim. Suleiman decided that rather than allow the Habsburgs to gobble up Hungary, thus making them able to meet the Ottomans somewhere on lower Danube, it would be to his best interest if the two empires met somewhere up the Danube. So he took Hungary.
Even then the Ottomans were operating at the very end of their supply lines. Troops had to be disbanded every winter only to be assembled at either Adrianople or Sofia. Then marchs up the Balkans, which, BTW took months. By the time the reached their destination, it was usually late in season.
So, my POD I is that Louis II didn't die at Mohacs. Suleiman got Hungary's neutrality in exchange for a stop on border raids. Hungary never divided. It served as the Shield of Christianity in the East. The Ottomans can now concentrate their power on Persia, Russia and Malta. Ottoman and Austria never met one another.
ALso, I disagree with the thought of fialure to conquer Egypt. It's very rich. It's the granary of the empire, like it had been under the Romans. The arabian provinces, Hejaz, etc was important because it contained Mecca & Medina. The Ottomans want them for prestige of Caliph.
As for Persia, the Treaty of Kasr-i-Sirin in 1747(please correct me if I'm wrong) established between the empires borders that changed very little until this day. Following the treaty, hostilities decreased considerable for a number of years. Therefore, I think the Ottomans can make lasting peace with the Persians.
POD II is that instead of Selim the Sot who inherited the throne, it was another son, Mustafa. In our TL he was murdered at the instigation of Roxelana, favourite concubine of Suleiman & mother of Selim. I read that Mustafa was Suleiman's best choice since he was capable. he apparently everything his father was and his brother not.
Okay, that's my opinion, thanks!
Cheers,
Rad