Xaden
I am reading fourth wing again for the second time and realised that many hints about the plot were given throughout the book.( I am sure you already knew this)
So I too have a theory about the upcoming book of the ending of this series which is- Violet will kill xaden. He has often mentioned how she will be the death of him of course considering the scenario in which it was said can mean as nothing more serious. However, I feel like their love is also too needy which can be toxic at times. Well whatever this is what I feel that they will die after securing everyone else.