THIS is how PLL should have played out/ended (in my opinion)
ENDGAME
Alison is A. She became A to find out who is responsible for the death of her twin sister Courtney.
In my version, the twins aren't enemies, they're incredibly close and their story is tragic. One twin didn't kill the other like in the books.
BACKGROUND
In my version, Alison and Courtney were raised apart from each other. For the first 12 years of her life, Courtney lives with their birth mother in Georgia (not Jessica - Kenneth cheated on Jessica for cheating with Peter Hastings. Very messy family. Ali and Courtney are the product of that affair.) and Ali lives with the Dilaurentises *Keep in mind Georgia is where Shana is from*
Courtney is placed in the mental facility, Addison-Stevens, in Georgia at age 13. That is where she meets and befriends Shana.
Two years later, when Courtney turns 15, Kenneth Dilaurentis decides he wants to be closer to his daughter (and he feels guilty for not being present in her life), so he transfers her to the Radley in Rosewood. Keep in mind, at this time Alison still has no idea she has a twin sister.
After a while, Alison becomes suspicious of her father, who is always sneaking around, buying secret gifts, etc. She suspects he's having an affair, but after snooping, discovers the truth about Courtney. Out of curiousity and disbelief, she visits Radley in secret and meets her sister. They form an almost instant bond.
Alison begins to resent her parents, especially her father, for keeping Courtney away from her all these years. She especially resents them for keeping Courtney locked away at Radley. Out of pity, Ali starts switching places with Courtney from time to time, to give her a taste of freedom. Courtney is a bit of a troublemaker while posing as Alison (she's responsible for the Jenna Thing, and most of the major scandals "Ali" stirs up) and Ali admires that about her. She's Alison's crazy other half - her soulmate.
Switching places is fun for a while, but the twins decide they'll never really be happy until they're completely free from Rosewood. So they hatch a plan to run away together.
THE MURDER
This brings us to That Night. The night of the barn sleepover was the night for the twins to execute the final steps of their getaway plan. Courtney is able to sneak out of Radley and the twins both wear identical yellow tops so they can be in multiple places at once without anyone suspecting anything.
Alison's job that night is to tie up loose ends - get the remaining blackmail money from Aria's dad, cover their tracks, and call their getaway car - Shana from Addison-Stevens.
Courtney's job is to stay with the Liars at the sleepover while Ali runs her errands. The sleepover is just their alibi for the night.
Once everything is taken care of, the twins are supposed to meet each other and finally drive out of Rosewood for good...but they never get that chance.
Courtney drugs the Liars so she can leave the sleepover without them knowing. Once she leaves the barn, she's immediately confronted by her killer and murdered.
Ali, after calling Shana, goes to meet Courtney and discovers her dead body. She grieves her sister, but has to act fast. Ali decides to bury her sister's body in the backyard, vowing that she will find the killer and avenge her sister's death. She is 100% convinced that it was one of the Liars.
THE A GAME
For her own safety, Alison returns to Radley, posing as her sister Courtney. All the while, she has Shana bringing her information about everything that's going on in the year following her sister's death.
Shana eventually meets Jenna and they bond over their hatred of the liars. Jenna introduces Shana to Mona. When Shana grows close enough to them, she reveals the truth about Alison and the four of them begin plotting.
*In my version, Jenna has been able to see out of one eye the whole time*
As soon as Aria returns to Rosewood, they decide to execute their plan to torment the four Liars. They are the original/main A Team.
THE KILLER
I would LOVE to hear who you guys think the killer would be in my version of the story!
Note: In my version, after Alison is revealed as the sympathetic "Uber A" in season 4 or 5 (Mona is still the first A to be revealed), the Liars accidentally kill her by pushing her off a building.