This raises all kinds of questions, because as we all remember he's the one that had the power to bring back Beric Dondarrion six times to life in the name of the Lord of the Light; Melisandre was absolutely astonished and shocked when she heard about this, as she knew that she herself would not be able to perform such a deed.
With the knowledge that Thoros of Myr will be a part of the story again, let's have a short look at this scene from the first trailer:
My first thought when seeing this scene was that it could be someone trying to resurrect Jon Snow, and obviously not Melisandre. I shortly considered Ian McShane talking about his character bringing back a "beloved character back to life that we think is dead", but I don't think he will be the one to raise Jon.
I can not confirm that Thoros of Myr had a tattoo on his hand like the person in the picture above - unfortunately he seems to have worn gloves most of the time in his season 3 appearances.
BUT if you compare that person with pictures of Thoros, it absolutely seems to be possible that he could be the one standing next to Jon; though not clearly to be seen, his hair, beard and even the structure of his coat might be very similar:
And most of all, it would perfectly fit into the storyline: Melisandre knows what Thoros has done before, so she might want him to try his resurrection powers again, this time with Jon Snow.
This could very well be the reason for Melisandre leaving Castle Black in Episode 5. Or Brienne travelling through the Riverlands could meet the outlaw band and / or the ex-warrior played by Ian McShane, who might have brought the Hound back to life in the meantime - well, wouldn't THAT convince Brienne that some people have powers to resurrect the dead?
So, if any of this should be true, we could have to wait quite a while for a resurrection scene with Jon Snow - and of course, noone knows if Thoros would even succeed IF he tried to raise him from the dead.
But there is also another possibility for a Thoros resurrection: We haven't seen Thoros for a LONG time and don't know where he is and what he has done in the meantime. But we DO know that Thoros not only is a red priest; he also has visions and sees things in the flames, like Melisandre. So he could be right on his way to Castle Black cause he might have seen something like the Azor Ahai being in desperate need for help...