I haven't read the books so no spoilers please, but knowing that Game of Thrones will continue for at least a couple more years would prevent Arya from being in the show as frequently as River right? Unless Arya dies next season (again, no spoilers!) I don't think we'll see her more than once or twice a year.
Something else entirely, I was watching the behind the scenes clips on the BBC website and there's a rather charming one of Capaldi celebrating his birthday during the taping of this episode:
http://www.bbc.co.uk/programmes/p035xf9n
He mentions that they did the same last year in a very similar setting (during the taping of Robots of Sherwood), and that 'hopefully he'll still be there next year'. He repeats that later when one of the crew members gives him a chair with his name on it as a gift with 'does that mean I'll be back next year?' (not literally what he said - I'm at work right now so I can't listen to it again, but that's more or less what he said. You can watch it yourself at that link above).
Could just be Peter joking around, but given the rumors about next year just having a set of specials I thought it was noteworthy. Seems like even Peter was unsure about what's going to happen, at least when they were making this episode.