So I was seeing a fan-made trailer the other day, and this theory (well, more like idea, since it's barely based on any actual clues from the show) came to my head:

What if during maybe chapter 4 of the crossover, we get some sort of final battle in wich Barry sacrifices himself in order to defeat the Anti monitor, therefore ending the crisis. Then, in Avengers Endgame style, five years pass, and everyone has moved on with their lifes as best as they can. But the Monitor, who had died in the crisis and took him five years for a new body to be created, contacts Oliver, who now lives happily with his family, but still feels tremendous guilt over Barry's death, since he was the one supposed to die in the crisis. The Monitor tells him that new threads are coming, and that he must travel to the past and change his life for Barry's one in orded for his world to survive. After thinking it for a while, he accepts, and contacts Wally to send him to the past. Once he hears Oliver's plan, Wally accepts. They both travel to the past, and while future Wally tries to stop Barry from sacrificing himself, Oliver does exactly the opposite. Unfortunately, they both fail, and just as the Antimonitor is about to kill both Barry and future Oliver, past Oliver realizes what's going on, and taking some sort of specialized unstable positive (or light) matter arrow that future Oliver lost during the battle, he stabs it into the Antimonitor's neck. But before the arrow kills him, and knowing the colission of unstable positive matter with antimatter would create some sort of explosion, he uses the antimatter to make past Oliver get stuck to him, and since Barry can't phase through antimatter (I suppose, it would be interesting if he actually couldn't), future Oliver drags Barry of the explosion radius before it blows, and after that, it explodes, killing both the Antimonitor and past Oliver in the explosion, as well as destroying the antimatter canon that the Antimonitor was about to activate as well. Since past Oliver sacrificed himself, tough, future Oliver starts banishing from the timeline, and *dies* surrounded by his friend and family. Finally, the crossover ends with future Wally returning to his time, assuming only his memories will get altered, and the funeral of Oliver Queen, in a similar way to how the chapter could start with the funeral of Barry Allen.

So that's it, I know, too detailed, extremely detailed, and probably not even something similar to this will happen in the crossover, since it's too similar to Avengers Endgame, but I like to think there could be a very small chance of this happening, and this way, all statements would be true, Barry sacrifices himself in the crisis (even if it's changed later), and Oliver would die in the crisis as well. So, what do you think?