One would think that the writer/directors need to extend things out a few years, if possible, while George keeps ahead of them as long as he can. If that's the case, I would expect to see the Dorne storyline with the Martells, Tyrion's adventures across the sea with the Targaryen boy, and hopefully the Ironborn storyline with Euron and Victarion. Cersei and Arya should keep them busy for at least another season as well. I'd say they probably have enough material for two more seasons right now, depending on how much they string it out.
If George can come out with Book 6 by the end of this year/beginning of next year, I expect that would extend the show another couple of seasons beyond that. Still not enough to account for Book 7, unless George has enough material right now for more than one book. I doubt that, though.
I have no idea what they're going to do in the TV show with Bran and Sansa, however, since both of those storylines are near the end. I can't imagine we'll see much of them the next season or two, unless they decide to push ahead of George with their storylines. Not sure I see that happening, not yet at least.
As for the Horn of Joramun, I believe Mance has it when Jon encountered him beyond the Wall, before Stannis showed up. He offered it in exchange for safe passage through the Wall. If not, Tormund was supposed to blow it and bring down the Wall.