Why would it just be fan service? It's not like Nymeria is an unlikable figure or something fans wouldn't want to see more of? Why couldn't it just be another loose end HBO is tying up for GRRM?
As much as the writing has suffered lately you have to at least give the writers credit for making some of GRRMs bullshit into tangible, quality on screen narrative. Hardhome, battle of the bastards, Brienne vs The Hound, the development of Bran and his greenseeing, they have nailed some aspects of the story and really salvaged the garbage that was books 4 and 5 and made it their own.
Edit: But at least if we never see Nymeria again Sylas can cover himself in his own glory for having made another prediction!