I'm pretty sure Blizzard wouldn't just bring Tandred back with no intention of using him. He's the only son left of Daelin Proudmoore and the brother of Jaina. Some of the most important Warcraft characters are in his blood and he's still in his prime unlike Katherine (but she still serves a purpose by being the one that brings Kul Tiras back in to the Alliance)
I just think there has to be something more to his return. We know he's an explorer (it says that on his "grave") so I think he found something that he couldn't tell anyone about because he was lost at sea. It could be the Dragon Isles, it could be Nazjatar or it could even be something Old God related but I think he knows something we are going to find out about real soon.
What do you guys think?