I really doubt it will be Big E. I think it's going be Cody, but it being AJ or someone from developmental isn't too far-fetched.
I think Cody is the best option - I wanted him to stay heel but at this point he needs a face turn because WWE isn't doing anything with him. Cody writing a letter to Kaitlyn rather than saying it to her face makes sense given their awkwardness and the fact that he'd be embarrassed that he let Kaitlyn slip from his hands because of the conniving nature of the Bellas.
The involvement of people like Big E and Rhodes could create other fun aspect to the Kaitlyn/AJ feud, so I hope the writers come up with some good stuff. Plus, Rhodes getting a face push toward the World title by feuding with Ziggler would be great, but I won't hold my breath for that.