Though the film may differ from the books, we can definitely predict "After Ever Happy" will see Tessa and Hardin's caustic relationship come to a head. They've had such a tumultuous relationship throughout the series, and it only intensifies.
At the end of "After We Fell," Hardin finds out that his mom is having an affair with an old flame, who just so happens to be Christian Vance, aka Tessa's boss and the man married to Kimberly, who's pregnant with their child. If that's not enough, Christian reveals he's Hardin's real father.
There's no doubt "After Ever Happy" will see those storylines through. The fourth book in the series also finds Hardin and Tessa becoming parents, so be prepared for that.