Okay, so if nobody is interested in going into speculations, let's see how you like the first draft of my idea.
November 10th, 1444, the Battle of Varna
The Hungarian king Władysław Itakes John Hunyadi's advice to stay put and wait until the army is reformed [PoD]. It does reform after routing the flanks and takes on the Murad II’s janissaries.
After a while, Murad II sees that he is soon going to be surrounded and flees.
The Christian army blocks the way to the south, so he goes to north along the coast. The Ottoman troop’s retreat is remarkably organized, the elite Janissaries hold the enemy back as the Sultan falls back and finally manages to escape on a ship.
Soon after that, George Kastrioti Skanderbeg manages to join the crusaders at Nicopolis and they lay siege on the city. They occupy it in mid-December and thus liberate Northern Bulgaria from Ottoman rule. Fruzhin, a Bulgarian noble, son of Ivan Shishman of the Tarnovo Tsardom had legitimate claim on the re-estabilished Bulgarian throne and was crowned Tsar on December 21st. He installed his seat in Sofia.
After the fall of Nicopolis, the Serbian Despotate and the Kingdom of Bosnia join the fight.
1445
March
Next year, the crusaders spend the first weeks of spring to secure mountain passes. In mid-March Serbian and Bosnian troops blockade Tirnovo, at the end of the month Plovdiv.
April
Hunyadi and Kastrioti march south. On April 2nd, 1445 the battle of Adrianopolis takes place.
The battle is fierce and bloody, but finally the crusaders prevailed. A plague epidemic broke out after the battle and decimated the local populace and the victorious army as well.
The news reached John VIII Palaiologos in Constantinople. He amassed a relatively small army and occupied Tekirdağ (Byzanthe).
May
After the plague took its toll, the Byzantine troops joined the crusaders and reconquered Gallipoli and a month later Philippi. Shortly after that Tirnovo and Plovdiv surrendered.
Most of the Ottoman holdings were now returned to Christendom. This was partly due to heir war with the Karaman Emirate.