Let me add on to the speculation. I think Vision is still alive in the mind stone as well. I think just like parts of all that is Vision was in him, I think that in his final moments he might have found a way to put himself into mind stone. Since it's unlikely Warlock plays the hero role in Avengers 4 like in the comic (which ****** me off really to introduce a character but move so far away for his main being), I can see Vision playing that role. He's already picked up Mjolnir. I'm pretty certain her can wield the power of the gauntlet.