I wonder if this is ASB. The Ottomans just seemed to consistently outclass European forces on land.
Perhaps some "act of God" (no puns intended

) might help the Crusaders, weighing the scales against the Turks. If the Ottoman forces are wrecked by disease or some natural disaster days before the battle (which itself might be an ASB premise, i suppose), the Hungarians and Poles - with some luck and better tactics - might at least obtain a minor victory.
Anyways, if any victory is obtained on the battlefield, unless its a decisive showdown, I doubt this does anything but delay the Ottoman advance. It might give Constantinople some time, but any Turkish ruler with enough willpower will try to at least besiege the city, and then it's up to the Europeans to march to relieve the city or let it fall. The Romans themselves had little chance of resisting alone, of course.
Even if it's a white peace or a less favorable settlement to the Turks, regarding the Europeans, if this butterflies away the death of Wladyslaw III of Poland, retains the triple unity between Poland/Hungary/Croatia, the Balkanic powers are in a much better shape to defend the survival of at least Bulgaria and Serbia.