"Game of Thrones" is synonymous with excitement and thrill and same applies to "Game of Thrones" Season 7. There are innumerable fan theories answering all your queries about the series but the questions never cease coming to the minds of its fans. Thus, it always leaves some scope for another new theory to be propounded.
In Season 6, Arya Stark avenged the death of her mother and brother by slashing Walder Frey's throat and crossed another name off her notorious kill list. This has triggered a new theory, whom Stark will kill in the "Game of Thrones" Season 7. GifHorsesMouth posits in Reddit that Arya will kill Melisandre as well as Cersei.
It is for sure that the Young assassin will continue her killing spree in "Game of Thrones" Season 7. The Avid followers of "Game of Thrones" series already know that in the Season 1 Arya was thrown out of King's Landing after her father's murder. From that point onwards she has started avenging for her family's downfall.
Rumors are rife about a war at the beginning of the "Game of Thrones" Season 7. Most of the major players will be involved in it, as per Bustle. But Arya's mission revenge may come at the latter half of the season and become a key point in the plot. Logically, to find out who Arya might kill, fans should consider that she might kill someone who poses a direct threat to the stark family and also easily accessible.
GifHorsesMouth's theory can also be taken for consideration. The theory concentrates on Melisandre's prophecy where she mentioned people with brown, blue and green eyes. While green eyes may belong to Cersei Lannister, blue eyes are Melisandre's own. Although nothing could be confirmed as yet, it is almost sure that Arya will bump into Melisandre in "Game of Thrones" Season 7.