It depends on how an epilogue would play out, but I agree with the OP that Arya and Sansa will both likely have open endings. I like the idea of Arya sailing west of Westeros, even if it's similar to LotR when Frodo leaves Middle Earth and sails west with the Elves. Also, considering GRRM has used Tolkien for inspiration, it would definitely tie in with the "bittersweet" ending he's been teasing. As for Sansa, I'm still debating whether she'll end up back with Tyrion. However, I imagine she could potentially go south in S8 because she has unfinished business with Cersei and it would allow for another storyline besides the war with the WW.