Yea the coalition that's coming will not be something that Cersei can solve by blunt application of violence. Tactically or strategically unless DD brings in someting from the book like the dragon horn. My prediction is that Dany will not have her fleet reach westros intact. For all i know, there could be storm, disease, sea monster and an attack by Euron's fleet.
Im also curious if Zombie Gregor got something to do with the white walkers. i got a hunch that Sam will find out Qyburn using cruder approach to what the children did for his creation.