1. Jughead is homeless and his estranged father is the leader of Riverdale's most notorious gang, the Southside Serpents. Veronica's father Hiram Lodge has hired them to wreak havoc in Riverdale in order to lower property prices so he can buy the town once he gets out of jail, and Veronica's mother is in on the plan.
2.Betty's older sister Polly had a nervous breakdown while dating Jason and Betty has a dissociative state when she doesn't take her meds and begins calling herself "Polly" and dressing and acting like her. Dark Betty almost kills Chuck Clayton in episode 3
3.Betty's mother hates Archie because he reminds her of Jason and is obsessed with destroying Archie before he can "corrupt" Betty. It is also heavily implied that she and Veronica's mother were in a love triangle with Archie's father back in their high school days.
4.The big twist is that Jason didn't die when he supposedly disappeared during his boat trip with Cheryl. He was held captive for roughly a week before being executed. Dilton fired the gunshot. He was illegally training his scouts to shoot. Also it's implied Cheryl and Jason were incestuous with each other.
5.Miss Grundy isn't the real Miss Grundy either, she's a con woman who killed the old Miss Grundy to steal her identity and lay low from the cops. This fact will be unearthed by Betty and Archie takes this very badly.
6. Betty and Veronica try boiling someone alive in episode 3.
7. The Archie/Veronica/Betty triangle will slowly build up throughout the first season. In the meantime Betty and Jughead will be dating by episode 8 and Archie isn't affected by this relationship in any way. Archie/Valarie (the other girl in the girl band) will also date. Archie/Veronica will butt heads over something.
8. Archie will be forming his own musical band.
9. Archie chooses Betty despite having feelings for Veronica.
10. Archie's father Fred once rejected Betty's mother Alice in high school. He is later arrested for Jason's murder after police determines it was his gun that was used to kill Jason, which causes Archie's mother Mary to return to Riverdale.
11. Betty believes her mother Alice killed Jason Blossom, but it was actually her father Hal, who wanted revenge against Jason for Polly's breakdown. He framed Fred out of revenge because Alice still had feelings for Fred.
12. Veronica's father Hiram has been working with Jughead's father FP Jones, a local gang leader, to lower local property prices so he can seize control of Riverdale. The family's butler Smithers has secretly been acquiring said properties under ghost companies and Hiram is ultimately released and return to Riverdale as the richest man in town.
13. Archie and the gang find out the Blossoms were occultists and Jason attempted to fake his death to escape from their influence. Polly had a breakdown after learning about the Blossoms's activities.
14. Jughead finds out that there are legends of supernatural forces in the woods surrounding Riverdale and that innocent blood spilled on Sweetwater Lake is said to be the key to awaken powerful forces. It is made clear that Jason being killed and dumped in Sweetwater Lake is a unwittingly a perfect reproduction of the ritual.
15. Final scene of the season is Sabrina ominously arriving at Pop's shop while Jughead reads about the results of the ritual in voice-over and how "chaos follows". She introduces herself and immediately afterwards there's a car crash just outside. Everyone steps outside to help while Sabrina flashes a smile.