If I had to guess, I'd say that they'll show Catelyn being resurrected at the end of episode 10. That's a scene that we only hear about later in the book, if I remember but for tv it would be much better to actually show Dondarrion giving up the last of his life so that Thoros could resurrect Catelyn. Otherwise, if they skip that scene and only reveal what happened later, like the book does, they end up having to remind everyone who Dondarrion and Thoros are, and how Thoros could revive people, when the viewer would not have seen any of that for at least a full season. Best to do it in this season, while all that knowledge is fresh.

How they use Catelyn and the Brotherhood between that point and when they intersect with Brienne is an open question, I guess. They might be able to Theon it, as you say, and show instances of the group taking revenge against Lannisters and Freys.