I can't see the Crusade of Varna being successful, even avoiding the battle, even avoiding litterally every failure the late Crusades met at this point.
Crusades, as an élan, a gathering of resources, supplies, men and (even if really relatively so) moral and politically will, was no longer : the rise of bureaucratic states in Europe more or less dried them out especially in a context were England, France, Germany and Italy (the traditional recruitment pools of crusaders) were already metting large scale wars (not that the XIth century was deprived of conflict of course, but it was what we would call low-intensity conflicts, which the HYW was not).
An idealistic crusading spirit still lived on in the Western world, true : Henry V Lancaster openly claimed he wanted to go to Holy Land and take back Jerusalem for instance. But it was just that : a political/moral fantasy without any real beggining of realization as much in the XVth than it was still in the XVIIth. At the very best, what existed were either campaigns with an admittedly important religious background but ultimately tied with nasceant national interests, either some foolhardy expeditions build on the declining social-cultural class that was the warring aristocracy to help some far states in Eastern Europe whom rulers never were able to contain or even lead at their benefit.
Does that means we couldn't see some late Crusade success? I wouldn't be so definitive : but it would ask IMO for an earlier PoD that would make either Hungary, Poland, or any athleta Christi strong enough to enforce their interest and viewpoint to a social class that was bent on its political and strategical independence (as Varna, and Agincourt both point very well).