Frankergregor will probably be The Mountain's reanimated corpse. Since he will be headless, he will be without any will of his own, more like a remotely operated flesh robot than a Frankenstein monster.
As for the girls that Qyburn "needs" for his studies of necromancy and golem-building, keep in mind that we already have a natural organ that is able to create another organs and human body parts: The placenta and the utherus...
Also, in another news regarding season 6 locations,
the GoT crew is pondering to film in yet another Spanish locale: Las Bardenas reales, a cold, desolate desert in the northern parts of Spain. I bet that the fact that there's the
Olite castle nearby also has to do with it too.
Keep in mind that it is yet to be confirmed, the HBO crew is just "scouting" the area, but I think that there are two obvious firm candidates for this type of location: A Dornish desert or... Asshai herself. The Bardenas Reales definitely looks more like real life Mordor than an sandy desert, me thinks. Judge for yourselves, guys: