I think this is the kind of reasoning by the crusaders that - flavored with "We've beaten the Ottomans, see the optimists were right" - is how you get an army convinced that they can beat anything in its path picking a fight with Timur and losing horribly.
The best the crusade of Nicopolis can do, IMO, is defeat Bayzeid and maybe return some territory to Constantinople's (among others) control - but "keep the Turks out of Europe" long term takes a revival of the empire of at least the 1330s preferrably with Venice and Genoa as help keeping the waters out of Turkish control, IMO.
I don't think returning territory to Constantinople would be a priority- if the ERE gets all of Thrace back, that would be a big deal. Certainly, the Kingdom of Thessalonica would be reestablished. Bulgarians would probably back Ivan Stratsimir, and they would definitely be the best off of all the Post-Ottoman states- though the Bulgarian state had been subdued, armed Bulgarian soldiers existed throughout its territory in garrisons under Turkish officers.
Also, I doubt the Turkish colonies in Europe would be as big a deal. Greece was still pretty urbanized so taking top-down control of it would be easier, and the settled Turkish ghazis would probably be unable to sustain resistance without an ottoman central authority.
Of course, all this is predicated on Bayezid and a large part of his army getting obliterated at Nicopolis. If they manage to retreat, they would quickly be able to make the situation untenable for the crusader supply line and reinforce through the aforementioned colonies (even if they were blocked off from Anatolia by the Italians).