The season 6 of GoT started with a bang with the return of peoples most favourite and ressurection anticipated character Jon Snow . This season seems to be favouring the Starks right from the starting with hinting events like escape of Sansa Stark , Arya being reminded of being a Stark by the lady crane as she tells she has very expressive eyes reminding Arya of her mother Catelyn, Bran saved by assumed to be dead Uncle Benjin (who is now cold hand), Jon getting the support of wildings and other small houses,etc. Would the show runners add return of Catelyn Stark too?
1) Brotherhood without banners sudden return!
The brothehood without banners wasn't seen anywhere after the third season and now in season 6 they make an unanticipated appearence and that too ending with death of bunch of common innocent people which is really against whatb they stand for because Ned send them in seaon one to save people in Riverlands from tyranny which was again told Arya during her encounter with the Brotherhood in season 3. Reason of them killing all the people could be they identified 'the Hound' and due to the revengeous feeling and lust of zombified Catelyn stark they would be helping her to kill all that betrayed her and her house. In books Catelyn is resurrected by the brotherhood after they find her dead near a river.
2) Lemoncloak aka Lem
Lemoncloak is among the prominent Brotherhood members that was seen in episode seven. He's a big deal because he's the guy in the books who introduces the Freys to Lady Stoneheart.
"She don't speak. You bloody bastards cut her throat too deep for that," he says, "but she remembers."
This post by the Actor himself signifing importance of Lady Stoneheart and teasing her return.
3) Bran's Visons series while Warging in season 6
Bran still warging when Meera was draging him away from the Night King's army towards south Bran saw young Ned's hands full of Blood , Mad king crying "Burn them all" , death of Catelyn Stark at Red Wedding , and pyromancer playing with the Wild Fire etc.. This is really where she comes in play I mean why show it all now ? This could be hinting towards her return as the incidence of the wrong alarm of return of Uncle Benjin just to kill Jon Snow or atleast attempted to ;).
4) Arya's thought of returning to Westros & Black Fish taking over Riverrun Castel
Arya just suddenly realising her House and what was done to her Family , When Lady crane told her that she had Expressive Eyes she suddenly seemed to be having a brain wave of all the pain she had endured because of the enemies of her House and their acts because of which she had to suffer so much. And the Sudden return of Black Fish (Uncle of Catelyn Stark) he was threatned by the death of Lord Edmure and was also taunted about where he was when her niece's throat was sliced open from ear to ear . Black Fish was very much determined to continue the fight while talking to Ser Jaime (Kingslayer).
5) Leaked Season Finale Synopsis
Till this date all the leaked episode titles and Synopsis of Game of Thrones has never been False. The season Finale Synopsis is
"Brienne meets a friend turned foe."
Well this doesn't seems to be pointing towards Jaime not that i could think of. So this might be pointing towards zombified Lady Catelyn Stark aka Lady Stone heart Because in the Books Lady Stoneheart is furious to find Brienne wearing Lannister armor and carrying Jaime's sword. She had ordered her to kill Jaime (for his failure to find and protect Sansa and Arya) or to hang herself to death for not fullfilling her oath.
I think this is some of the points pointing towards the return of Lady Stone heart and there are also some other good clues showing her possible signs of return click here. And if you have any other suggestions do tell in the comment .