As we know, Genoa did put harsh constraints on the Byzantine Empire, including fleet basing rights, but I think Venice might have even tried to conquer the Byzantine Empire! If that true, then it would have made it much harder for the Ottomans to later expand out, since the Byzantines were near collapse in 1420 when they fell, anyway.
If Venice had managed to conquer Constantinople, and convert it to a Catholic State, this might have been enough to completely butterfly the Ottoman conquests, or at least push them back to about 1500. By that time, maybe Austria could have expanded into the Balkans?