I think Daenerys will possibly end up pretty unhinged, either by the revelation that Jon Snow is the true heir or because of some huge loss (her dragons?).
I don't know, I think Cersei is definitely headed in that direction. She's lost everything that means anything to her and everybody has abandoned her side. I think Tywin was spot on when he said that she thinks she's far more clever than she really is, because all of her plans have led her to total isolation and pretty much a doomed existence. She's totally consumed by vicious hatred and revenge and that could potentially drive her to total madness when she realises that she is going to lose.
Think about it, she's lost her children, her family are all either dead or have turned on her, and she's back to having to sleep with men (Euron) so she can cling to some sort of power. I honestly think she's at a point where she's lost all genuine grip on the reality around her (that moment where she's muttering about elephants after Euron and her do the do was strange) and can see her becoming even more erratic and cruel as this season goes on.
She is a good character though, and I do have sympathy for her. Tywin was obviously a completely unloving father who gave all his attention to Jaime, and she was married off to an awful man at a young age. It's completely understandable why she's become this awful person, very much a product of her environment and upbringing. She's experienced so much trauma though and I can simply see her unravelling as the season goes on, perhaps Qyburn will slowly start to realise what is happening and abandon her.