Ever since the fourth season of arrow started , people never stop thinking about who is in that damn grave , the last episode left us three Candidates , first is William , donna smoak , third is thea.
First candidate william (90%)
In the ending scene of episode 14 we saw Damien dahrk kidnapped William and told his daughter to play with him , so everything appears normal.
Did you see how Damien dahrk reacted when he knew that green arrow knew he had a daughter , he also knows the value of having a child , so that's the only way to break Oliver's heart , or he wants to torture him , but killing William is something merciless , something oliver will never forget.
Oliver and felicity break up , but why was felicity so sad saying : "you have to kill that son of a bitch ".
Second candidate Donna smoak (5%).
Why killing her ? To break lance's heart , will dahrk know that lance is working with green arrow , or she will be killed accidentally.
Third candidate is thea.
Killing thea is not easy , because Damien dahrk's magic is ineffective against her , but it will be a taugh enough action to motivate oliver and felicity , killing green arrow's partner and Oliver's sister will not be melancholic to only us but the whole team and friends , even Barry Allen aka the flash.
No one could be in the grave
This is the least possible thing that could happen , could oliver fake the death of his son to protect him or the writers will break our heart and make Damien dahrk kill the innocent child.