This debunks the Stannis theory:
- Brienne does 100% kill Stannis in that scene. The only reason they didn’t show him actually dying was to avoid being gratuitous.
(Plenty discussion on that article on /r/asoiaf. Head over there if you don't mind some potential book spoilers - the few still left.)
Beware: I'm saying the following in this section based on casting info and set leaks, I don't know if you follow them, but can't answer without them~
I don't see Boltons stitching Stannis' head back over his body. Plus, that shot has the Bolton cross with army behind it - looks like it's before(?) a big battle in the show, likely somewhere around E09 or so. That means the timing is likely weeks/months removed from from Stannis' death, which means he's not a very impressive looking corpse anymore.
I think it's either one of Stark loyalists, or... dons tinfoil Ramsay? Roose? I don't know, Ramsay being all WINTERFELL IS MINE makes me think he went off the deep end (he was close already) and killed Roose what with a new and more legitimate baby Bolton on the way. Or Ramsay tried, Roose stopped him, and it's Ramsay on the cross.
As for Sam... hm. He's heading into new place (Oldtown, wider Reach?) and new characters (maesters of Citadel, his family?). New characters might not be very interesting to watchers, at least not in a trailer. Then maybe they still haven't finished CGI on Hightower? Or maybe the footage is super-spoilery. ¯_(ツ)_/¯