The witch's prophesy said that Drogo will return when:
"When the sun rises in the west and sets in the east, When the seas go dry and mountains blow in the wind like leaves. When your womb quickens again, and you bear a living child. Then he will return, and not before."
The sun rises in the west and sets in the east could be a reference to Quentyn Martell (Martell sigil being the sun), he came from the West but died in the East.
The seas go dry could be a reference to the Dothraki sea, which is drying out and the grass is dying.
The mountains could be the pyramids in Meereen collapsing after Dany's dragons destroy them. (Although I wonder if it could refer to Gregor Clegane in some way as he was known as the Mountain that Rides).
At the end of ADWD Dany gets her period, but she isn't expecting it, which has led some to speculate that she is having a miscarriage and therefore her womb has "quickened" again.
So Dany could well have a living child at some point in the future and Drogo will return (zombie-like maybe? Like Catelyn or Dondarrion, or she could just name the child Drogo?).