We have seen in the past that the flash can do marvelous things with his powers. Such as going back in time, creating a thunderbolt, can run though walls etc.
Oliver needs the Barry's help to save felicity. Barry can can go back in time stop Darhk men from hurting felicity and Oliver.
Could this change alot of things? Yes one big thing is that Oliver might not kill Darhk. If you saw the tailer Oliver said he was going to kill him aka Damien Darkh Witch could be really bad. Darhk is to dangerous to keep alive.
If you agree with me tell me in the comments. If not what's your theory.