I do agree with other editors that hope is there for the finding, if we manage to look hard enough. Certainly I feel that Kate Keller, once she has recovered from the death of her husband, will be a happier person now that she has given up the illusion of the return of Larry and also can be open about her husband's past. Likewise Chris and Ann are free now to marry and hopefully make a better future, having learnt something about the past.
In addition, the play ends with the hope that the negative experience of the war may have a positive outcome on the way people regard each other. Chris says that those soldiers who were selfish made it back alive and the ones who died were the ones looking out for each other. Similarly, Joe's death is prompted by his realization that he did not consider the negative effects that his decision to ship the cylinders may have had on the soldiers who eventually flew the planes. These examples suggest that we need to be more considerate of the effects that our personal actions may have on others--sometimes these consequences prove fatal and there is hope that we realize this.
The play ends, as do all plays written in the Selective Realism style, without a lot of closure. It leaves the resolution open to the viewer/reader to determine what might happen next. Even more importantly, it leaves the viewer/reader to determine how they might live their lives differently, to learn from the mistakes of the Keller family. Arthur Miller wrote many of his plays with the idea of the audience, along with the principal characters, exploring their motivations, actions, values, etc. You might call this self-exploration.
So, the play ends with Joe committing suicide and the other characters in action around that terrible event. The hope is that perhaps Kate and Chris will develop a real, based on truth, relationship. There is also hope that Chris, who according to Jim, had given up his idealism, may have a hope of retrieving some of that and not become a bitter, angry man.
It's hard to imagine there being hope for Chris and Ann's relationship, but you could make a case for it gaining strength from the truth being finally out. Perhaps if Chris is able to help Steve once he is released from prison, that would leave some hope for healing between the Deever and Keller families.
There is hope that two fractured families will reconnect. The Kellers, Chris and Kate, mother and son, will have a more honest, loving relationship and the Deevers, Steve, George, Ann and possibly Mrs. Deever will have a chance to heal their family for a more positive future.
There is hope in the fact that the truth has been uncovered and that an innocent man will now be set free from jail. Steve Deever should be released from prison, once the truth is told. He can recover his family, who up until this point has not wanted anything to do with him.
Kate Keller finally comes to terms with the fact that her son, Larry is dead. This is a positive step for her mental health. Additionally, Ann is now released from her promise to the long dead Larry, now she can move on with her life, probably not with Chris, but with someone else. Chris, who survived the war, I think will have a better relationship with his mother.