I haven't been watching but a friend of mine pointed out that Flash's whole season is built around Nora coming back in time because Barry just disappeared. Sounds a lot like what happens to Flash/Barry in the Crisis Comics.
Which now has me wondering if Flash and Arrow have been setup as a 2 year story leading up to Crisis? Maybe the future timelines where SC goes to hell and Flash goes missing are all related to Crisis eventually happening?
If that's the case then I suspect you'll get a reversal at the end of crisis which changes the future and that's due in part to Nora. So original timeline Crisis happens, Kara dies, Barry disappears and maybe Oliver dies too. This leads into Crisis which ends with Nora magically changing timeline to bring any combination of Kara/Oliver/Barry back to life.
ETA: the whole point of Crisis on Infinite Earths was to destroy the Multi-Verse. Makes me wonder if Arrowverse Crisis will be used to being Black Lightning and Supergirl into Earth-1?