Well, if I was assigned to write it....
Early on, maybe right at the start of the episode: The morning after the RW, the Freys are celebrating and mopping up. We see Robb's body with his wolf's head sewn on, Catelyn dumped in the river. Flawless victory for the Lannisters. Sad, sad, sad.
Follow somebody else for a while... then back to...
Arya's vision where she wargs into Nymeria and pulls her mother's body from the river, then flees when the Brotherhood arrives.
Alternate, just show it straight rather than through a vision - but the vision is a nice way to tie it to Arya, whom we also need to see in this episode. So let's make the vision the beginning of her segment, then she wakes up from it - what did THAT mean? And then we resolve her adventure with the Hound and she's done for the season.
Follow others again for a bit...
End of episode, The Brotherhood around the fire, doing a ritual. We're not sure what it's about at first... mumbo jumbo mumbo jumbo... holy shit. Zombie Catelyn WTF
End credits.