Fergus in a heartbeat. The Couslands have already been contenders for the throne, and I fully expect the family to take over rulership of Ferelden when the Therin line ends with Alistair.
God, I hope not. I'm so sick of the game practically giving the Couslands a hand job for being the Maker's greatest gift to Thedas since Andraste just for existing. If they took over the throne after Alistair and/or Anora stepped down, regardless of which Warden became the Hero of Ferelden, we would never hear the end of it from Cousland fanboys and girls expecting everyone in the forums to gush over the Cousland PC and family as much as the characters gush over them in the game.
Besides, I remember how elves were treated in Nan's kitchen in the Cousland Origin, and how her verbal abuse and threats were treated as normal, if not endearing. I wouldn't trust them to help with the elves' plight.
Ideally, years will have passed after Alistair and/or Anora drops off. I hope someone else becomes just as if not more qualified to take the throne by then.