I Think either a Hillary Clinton/Jeb Bush or Jeb Bush/Hillary Clinton ticket would be hard for republicans to beat.
It is already difficult to distinguish Dems from Pubbies.
Recall that in 2000, McCain threatened to jump parties.
Recall that in 2004, McCain wanted to be Kerry's VP.
Recall that in 2008, McCain wanted Lieberman as his VP, but his staff balked.
If Obama were running again, he could select Boehner as his new VP.