I don't see the Others storyline as one that's ever going to involve all the characters directly. This doesn't feel like a series that's building to an epic confrontation with humanity unified against a monstrous onslaught. In fact, the relevant prophecies and visions suggest it's individual players (Jon, Dany, perhaps Tyrion and Bran) that are important rather than rallying the troops. Probably the threat will manifest in some way that makes all the characters aware of it, creating a sense of finality and moment without requiring Arya, Sansa, Brienne, Jaime, and other characters with no evident connection to that storyline to be shoehorned in.
"The main story" is the chaos that spreads across Westeros during the timeframe of the series, its effect on the central characters, and what that reveals about human nature as constrained by medieval values and resources. It's not so tightly bound by the unity of action that we have to pick a particular narrative strand as the main one.