Who do you think will eventually end up on the Iron Throne?
Well, in my headcanon I personally would love to see, after defeating the Others/Wights and taking down the dangerous people in that world, Daenerys reclaiming the Throne, but ultimately will have a joint-ruling with Jon Snow while appointing Sansa Stark to be of assistance as well, including her being the rightful heir as Ruler of the North. Arianne Martell and Margaery Tyrell will also join Daenerys rule the Seven Kingdoms and create a democracy all across Westeros.
But in terms of book canon, I believe the Iron Throne will become obsolete in the end. There's been so much death and betrayal over who gets to sit on that Iron Throne that it wouldn't matter anymore. There are bigger threats to deal with and whoever is left alive in the end will probably want the Iron Throne destroyed, realizing that it would only create more problems with history repeating itself all over again. Unless the series ends with a pessimistic tone, which I'm really hoping not.