Hi everyone! First time poster here, long-time reader. I just wanted to add my two cents about the role of Gendry and, in extension, the romance-for-Stark-sisters-theory. I believe the lack of obvious/direct reference to their expected ultimate love interests (Gendry for Arya and Sandor for Sansa) is a result of a few different factors:
1. Both Maisie and Sophie were, until very recently, basically children. If not legally, at least in our minds they have been. I think the showrunners were therefore happy to let them play out their other arcs and wait for the final romances - with grown-ass male actors - until the final season.
2. In the meantime, therefore, they have developed and completed the romantic arcs of the adult pairings. Dany/Jon, Missandei/GW, Jaime/Cersei etc. The only one not yet resolved completely is the Brienne/Jaime one and even that can be tied back to the Stark sisters. Brienne has been Sansa's and, now, also Arya's protector/parent-figure. That was her primary role. But this season there has been very pointed references to the fact that the Stark sisters are not children anymore (Arya's pretty, you guys! Hot Pie says so!) and can take care of themselves. Sansa even literally sends Brienne away to KL where Brienne has an "aw, look how much they've grown!" moment with Sandor with regards to Arya. I get the feeling this repeated emphasis on the Stark sisters' transition out of childhood and into adulthood is for a reason - it subconsciously primes the audience to be fine with seeing the girls have romantic relationships with their adult male co-stars. With the adult pairings - esp. the big Jon/Dany one that they devoted the entirety of S7 to - done and out of the way, the final season is open for romances to develop for the Stark sisters and, by extension, for Brienne and Jaime to complete their arc (be it a happy ending or not).
3. Littlefinger. He really needed to be out of the way before Sansa could move on in her personal life. He's been a leech and emotional abuser - beautifully and subtly indicated by Sansa's single tear as he's executed. Like it or not, he had some hold over Sansa emotionally (I don't believe she loved him but there was some dependency there) and in S7, Sansa finally confronted that unhealthy situation and had him removed. Now, in the final season, I believe she will be given the space to heal emotionally and open herself to more healthy relationships with men who are not trying to manipulate her and will never lie to her. In fact, Sansa even directly revealed her desire to be around forthright people this season when Arya was being evasive about the whole "You took mum and dad's room" issue and Sansa tells her to stop doing that and "Say what you mean". Well, guess who never holds back from saying exactly what he's thinking??
4. Similarly, Arya has been driven by anger and bitterness for so long that her arc will not be complete until she finds some emotional healing (or dies, but that's another story). This season was a step in that direction where she patched things up with her sister (although the execution of the arc was deplorable, IMO). It is only good storytelling to end her arc next season by finding some sort of emotional resolution. This might be where Gendry comes in. He reminds her of the true friendships she's had in addition to basically having been her sexual awakening back in S2.
Ultimately, GRRM was going to name the last book in the series 'A Time for Wolves'. Even though this might not be the name anymore, I think it gives us a hint of the flavor of the last chapter in this saga. The ending, I think, will be in large part dedicated to the Starks - which is basically just Sansa and Arya at this point - prevailing, both, in terms of the game of thrones and also as individuals with emotional lives of their own and as the parents of a new future after this story has ended. This is why, I think, we've had the characters of Gendry and the Hound brought back to the show when, really, there was no other real reason to add them to the payroll again.