Surely the death knell for the Liberals comes if the Tories choose a socially liberal leader?
Other than Brad Wall, I can't think of any (potential) Tory leader who isn't encumbered by their association with Harper. The Liberals (helped indirectly by the NDP) will gleefully point these connections out.
A real danger for the Liberals is if the NDP continues to Blairize under Mulcair and in government. The Liberals will be squeezed on the left and unable to catch Red Tories without alienating its centre-left wing.