Some might take that to mean Theon, Yara, and Euron will be insignificant in the final six episodes of the show. But, given how tightly locked down this season has been to prevent spoilers, this absence could mean they are actually very significant — but showing them in the trailer would reveal too much about how they factor.
The “Golden Company” theory
Fans of the book series also think Theon could help bring the Golden Company into the mix. This company of sellswords led by “Homeless Harry” Strickland is 10,000 strong, with horses and freaking elephants. They’ve had a few brief mentions in the series but we haven’t really seen them. In Season 4, Davos Seaworth suggests Stannis Baratheon engage them, but Stannis balks.
In Season 7, Cersei says she has hired the Golden Company and has put Euron in charge of bringing them to King’s Landing.