Concerning a Bill Clinton Third Term as President:
As mentioned he could run for VP and then the President could resign after taking the Oath of Office. This is unlikely to happen.
Scenario that might work:
2008 Dem. Ticket Clinton & Clinton. Hillary as Pres & Bill as VP.
Don’t discount this. I think among moderates and liberals, Bill’s appeal is growing. Many would say he was an excellant President on policy and had some self control issues.
Plausible is not highly likely. They would then effectively work as co-presidents.