I'm calling it. John will be falsely accused of Hal's murder in 2016 and will uncover the truth about his whereabouts in 2026 to clear his name.
Many people assume Hal is dying, but they overlook that the show is deliberately misleading them this way. It feels too obvious:
My theory is that the show will literally start in 2026 with John in prison for the 2016 murder of Hal Jordan. This explains why he wasn't in Superman 2025. Similar to season 1 of True Detective, new evidence will reopen the case, and John will be asked to revisit the 2016 case from Nebraska, before Hal went missing. This will be the season's main tension, as viewers wonder what ultimately happened to Hal between 2016 and 2026. The tensions between John and Hal will increase, and quotes like Laura Linney’s "then go get it, John Stewart" or Hal’s "over my dead body" will serve as red herrings. In the end, John will escape prison, and the 2016 case will reveal Hal's whereabouts in 2026.
Whether it's Parallax, Spector, or something else, I believe Hal will survive, and this is all a clever bait-and-switch.