If this were Terry Pratchett, I would imagine that Catelyn and Benjen would lead an Undead Liberation Front and take over the far north as rulers. As it is, well, what do you do with kinda dead characters?
Benjen may be warged into the crow, now, though?
I suppose one problem with Benjen having a starring role in the outcome, is that GRRM seems to be clearing the deck for the next generation characters. But I would not surprised if he turned up to pull off something significant and then perished. Otherwise he is a loose end.