1. Arya will kill Cersei with Jaime's face 

Having Jaime kill Cersei would just be too easy. It wouldn't surprise anyone. However, such an act would properly fulfill the Volanqar prophecy. That said, given that Arya has only used her faceless men powers once, which was to target and assassinate Walder Frey, it could be reasoned that her assassin character arc isn't quite over.

Redditor greypiano predicts that Arya will be the one to kill Cersei. If she first kills Jaime, she can use his face to get close enough to Cersei to then kill her. That said, although this wouldn't neatly fulfill the volanqar prophecy, it would still complete it.

2. Cleganebowl will happen 

Cleganebowl is sure to happen this season. The previous seasons have set the two brothers, Sandor (The Hound) and Gregor Clegane (The Mountain), on an almost certain collision course with each other. Currently, The Hound is fighting alongside Jon Snow and the Northmen, while The Mountain is Cersei's puppet and does whatever she commands him to do.

This theory suggests that The House and The Mountain will meet at one point and fight to the death. Hopefully, as most fans have demanded, Sandor will be victorious in taking down his brother once and for all.

3. Cersei's repayment of her debt to the Iron Bank will lead her to her downfall

Tywin Lannister always owed a debt to the Iron Bank. However, he did this on purpose as he knew the incentive of repayment would make continuously back him. Of course, Cersei was arguably never as smart as her father. With Cersei repaying her debt to the Iron Bank, the incentive for the Iron Bank to support and be patient with her is gone.

4. It will reveal the significace of the saying "There must always be a Stark in Winterfell" 

There has always been a Stark in Winterfell since the beginning of time. Could this simple fact be why there must always be Sark in Winterfell? It's always been thought that this was just an offhand remark and that there was no actual reason for a Stark to remain in Winterfell, but rather had become precedence to follow.

But what if there was a mystical explanation for at least one Stark staying in Winterfell?

This theory suggests that the White Walkers wouldn't have come back to life if a Stark had stayed in Winterfell. According to some fans, there is a hidden meaning in the saying "There must always be a Stark in Winterfell." Moreover, fans argue that the saying is more than just a line or a superstition since it's believed that if no blood members of the Starks stay in Winterfell, bad things will happen. Like awakening the Army of the Undead bad.

5. The final scene will reveal that Samwell Tarly has been telling the story of Game of Thrones

In season seven, Samwell Tarly and Archmaester Ebrose had the following conversation at the Citadel library:

Ebrose: "If you're going to write histories, Tarly, you have to do the research. If you want people to read your histories, you need a bit of style. I'm not writing The Chronicles of the Wars Following the Death of King Robert I so it can sit on a shelf unread…

[Sam shakes his head at the title] Ebrose: "You don't like the title? What would you call it then?"

Sam: "Possibly something a bit more… poetic."

I don't know about you, but the titles A Song of Ice and Fire and Game of Thrones sound poetic to me.

Redditor u/thetripleb theorizes that Sam could turn out to be the writer/narrator of A Song of Ice and Fire (book series)/Game of Thrones (tv series). It's speculated that the last scene in the final episode could end with "...him putting the book away after telling the story to someone, maybe his grandkids, and he'll be played by GRRM."

That said, it would make sense for George R. Martin to play old Sam since he has often compared himself to Samwell on more than one occasion over the years.