In march 2016 the Shinkansen will link Honshu to Shin-Hakodate. According to JP Rail's website the Hokutosei is already discontinued and Japan Guide says the overnight Cassiopeia and Hamanasu services will be discontinued in 2016.
If this chart from Wiki is to be believed the journey will still take around 7+ hours to Sapporo from Tokyo.
Are there any plans to maintain any overnight train service to Hokkaido beyond march 2016 ?