Warning! SPOILERS ahead for The Walking Dead midseason finale and comics.
The Walking Dead season 9 midseason finale leaves Negan with a huge opportunity once he escapes prison, and however he chooses to handle this new situation is sure to impact where the series goes next. And Negan's dilemma isn't the only cliffhanger, meaning that when The Walking Dead returns next year, season 9 will still have a lot that needs resolving.
Negan is a very different character in The Walking Dead season 9. He's been imprisoned in Rick's basement ever since the war ended and the captivity is driving him crazy. When Maggie finally confronts him (before The Walking Dead's six-year time jump), he's a complete wreck, breaking down and begging for her to kill him. After the time jump, Negan appears more stable, receiving counseling from Gabriel and regularly chatting with Judith. Still, he remains isolated, unable to stir up the masses and, therefore, rendered effectively powerless.
In The Walking Dead season 9 episode 8, "Evolution", Gabriel is again visiting Negan in his cell, but during this particular chat, Gabriel is distracted, still reeling from the news about Rosita. When he leaves, he forgets to lock the cell door, and, during a storm later in the evening, the rattling of the lock clues Negan in. He walks up to the door, pushes it wide open and steps through - an all too familiar mischievous and smug look on his face. Negan is free, and what he chooses to do next could change everything.
What Happens After Negan Escapes In The Comics
An almost identical scenario also happens in The Walking Dead comics when Negan's cell door is accidentally left unlocked by Olivia after she gives him a haircut. She, like Gabriel, was also flustered by Negan, and, in wanting to just leave as quickly as possible, doesn't take the time to check that the gate is secure. When Negan later leans against the bars, the door swings open, and his escape seems all but guaranteed.
However, Negan does what nobody would expect - he stays! When he's later visited by Rick, he makes a huge deal of the fact that he chose to remain even with his cell unlocked, urging Rick to consider it a sign that he can trust him. Rick doesn't buy it, of course, and Negan is again locked in his cell.
But Negan does eventually and successfully escape with the help of a teenager who holds a grudge against Rick. Once outside of Alexandria, Negan actually joins up with the Whisperers for a short time. But after realizing there's no way he could ever convince any of them to follow him, he kills and decapitates their leader, Alpha, with the hope that, when he presents her head to Rick, he'll finally believe that he can be trusted.
What Will Negan Do When Season 9 Returns?
When it comes to what Negan will do when The Walking Dead season 9 returns in 2019, the TV series has two options: either adapt one of the two scenarios above, or do something completely new. At this point, it seems most likely the show would follow the comic's example and have this whole tease of Negan escaping be just that... a tease. It'd make sense that when the TV show returns next year, Negan will be discovered sitting in an unlocked cell, hoping his choice to stick around will have earned him a little trust. (And a possible release for good behavior.)
On the other hand, the TV series is already starting the Whisperers arc, which means it's possible Negan will take this opportunity and make a break for it, possibly running into the Whisperers. He clearly plays a significant role in that storyline in the comics, and since The Walking Dead has already jumped ahead in time, there's no reason they can't just advance Negan's arc as well.
In the end, it's possible The Walking Dead will take Negan in an entirely different direction. Maybe he'll arrive in the nick of time to save Daryl, Michonne, and the rest of the group? Maybe he'll just take off and live like a nomad (which is something else he eventually does in the comics)? Whatever it is Negan decides to do, it's sure to have a huge impact on where The Walking Dead season 9 goes next.