Good question, I personally believed that the most significant event would be Talos becoming a god. The Talos Ascension is a huge game changer. A mortal man becomes a god, as in, superior to all. The elves, who believe themselves superior to man, now are inferior to man. This completely flips the scales and has huge impacts. Also the Dwemer vanishing to another plain is a massive event. An entire race, that stretches across Tamriel, vanishes all at once. I don't even know where to begin if they stayed around. An honorable mention is the wiping of the existence of Jungles in Cyrodil across time.