6. Battle Of The Greyjoys
Towards the end of Season 6, Theon and Yara Greyjoy arrived in Meereen, pledging loyalty to Daenerys Targaryen, and in return she would - if and when necessary - support them against Euron Greyjoy.
With the Crow's Eye on their tail, it was widely suspected that the first major battle of Season 7 would come at sea. As Daenerys heads to Westeros, she'd be met by Euron's ships, with the Greyjoys engaging in battle.
It's not clear to see if Dany is involved at all here, with the focus on her being in Westeros. But we do clearly see Theon Greyjoy involved in the sea-battle, as well as another shot of the fiery wreckage, a shot of Euron's ship The Silence, and two people (one of whom is probably Theon, and the other could be Yara) diving into the water to escape the fire above.
So yeah, we're definitely getting a Battle of the Greyjoys early on in Season 7, and it's one where we could well lose some people.