One night To 300 years later JELSA

Elsa was 17,Jack was 18 both Human, Elsa still had her ice Powers, Jack didn't YET! the two were practically dating, what happens when One night on a frozen Lake Jack and Elsa kiss, But then the two fall in the lake, drown and die together?, It's 300 years later and Jack is a guardian,And Elsa lives on the North mountain Alone, What happens when Elsa is chosen to be a Guardian?