What of the Ottoman Empire's forces had been defeated by the crusaders at the Battle of Nicopolis
The main problem is that, right from the beggining, the whole Latin army was desunited into various groups which didn't have even half of a plan in mind.
While technically at the head of the army, Sigismond of the Hungary had no real authority beyond his own men and had to, several times, give up before the opposition of other great nobles, especially the French armies.
It did not only provoke an important rift between Latins, but as I must stress, also that they had no common plan for the expedition or the battle apart "staying there and see what happens". They litterally brang no siege weapons with them, forcing a long siege before Nicopolis, and made no scouting (up to really late, they didn't believed Bayezid was in Europe).
In these conditions, a stalemate was all that Latins could really hope for, allowing them to withdraw in order. And that alone would be a tall order.
All of this could be challenged, of course, but would require an early PoD probably implying several changes in France in order to make the situation way stabler (probably trough an even more decisive
Caroline phase) and eventually allowing more contingents from France and England. You'd probably need to make Charles VI way saner, as to preventing a too great social-military importance of great princes and more men of the king in the whole expedition (especially with an earlier peace with England).
Having more English, and if possible Italians but that would require even more changes in Europe and probably another PoD, to balance out the influence of French nobles would be probably interesting to allow Sigismund and his Vallach allies to impose their relatively more sound plan of battle (relatively because of aformentioned issues).
And even there, I don't see a real decisive battle for what matter Balkans in the long run, but arguably you could see Sigismond gain some presence beyond Danube. But the Bulgarian Empire was already crumbling, and the Ottoman focused on this region to crush down every Christian coalition they could met : they had the logistic to resist foreign expeditions way more structured than IOTL crusade, and would return with a vengence soon afterwards.