Rubio - nobody, race will be decided on election day 2016
Cruz - Pete Sessions? George P. Bush? Many possibilities here.
Longshots
Paul - Edelen under Conway, Massie under Bevin.
Sanders - Welch under a Democrat? Under Phill Scott? I don't even know.
Graham - Not happening, but it would be Gowdy or Mulvaney
If Sanders were elected President/VP, wouldn't he just resign the senate seat in December to ensure that Shumlin appoints his successor?
Of course if a Republican is elected governor, he'd resign while Shumlin is still governor so that a Democrat could be appointed.