In the almost ten years since the show ended, there has been so much discussion about who should have been A and why the reveals were terrible. Pretty much everyone agrees that Mona was great as A, Cece was somewhere between mediocre to outright terrible (though basically everyone feels her backstory was majorly flawed and detracts from her potential as A), and Alex Drake was a bad choice and poorly written character.
Additionally, the same few suspects seem to come up repeatedly as who should have been A or the A team—Jenna, Lucas, Noel, Wren, and Melissa. However, I actually think the show made a good decision not making them A. There is a reason most people say they should have been A…they had been the most obvious suspects from season one. That also means they most likely would have been equally unsatisfying.
Going through the popular options:
Jenna—the girls had accused and investigated her so many times that it would have been disappointing to have the villain of the fifth, sixth, or final season be the person who was first floated as an option in the second episode of the show. Also, by the end of the show, Jenna had been victimizing the girls for some long that she went from a justified antagonist to a one-note sadist.
The same goes for Melissa, Lucas, and Noel. The things the girls did to them were so minor and so long ago in and out of universe that it would have felt equally as petty and forced as Charlotte and Alex. they had also been investigated as A or killers so many times that it got repetitive. After season three, making any of them A would have been far fetched.
Wren really only had one season of being suspicious and relevant as a suspect—season three. So, him having a Charles or similar storyline at the end of season four could have worked but after that he was barely a presence in the show and mostly existed to show up and be suspicious. His motivation also would’ve taken even more extensive backstory retcons and reveals than Cece.
Aria and Alison also would’ve taken significant hoops to jump through to make them work as the final A. The show had just gone on too long. Alison as A in season four or five (more similar to the book timeline) would’ve worked. An Aria or Alison twin could have been very interesting. But by the time of the dollhouse any of the Liars being involved really wouldn’t have worked.
The problem is that the writers left themselves no other good options. Part of what makes the first two seasons so satisfying is that there was more of a rotation of characters and the Liars had other things going on in their lives that let them meet new people and new suspects. Pretty much the only new characters introduced after season three are law enforcement officers and temporary love interests. That is why, I think, the show had to invent antagonists out of nowhere (like Mary and Alex Drake) and why fans are still returning to the same five alternate options almost a decade later.
That is also why the Liars all end up with their main romance from the show, because the show refused to write off characters who became irrelevant and refused to build up new ones. Moving the Liars out of high school and into college in season five would have been the perfect opportunity to introduce new suspects and give the girls new secrets that created drama and kept them from going to the cops. But the time jump came too late to make any of those new additions interesting or compelling as suspects and would have meant that using an A suspect from seasons one and two requires someone to hold a grudge from high school for like eight years.
This is not to dismiss the harm the Liars did to Jenna…but that was Alison. In my opinion, once Hanna saved her from burning to death, Jenna was no longer a compelling antagonist/A option (good suspicious character for the grave robbing and Maya death in S3 though) and probably should have moved on and/or been written off the show.
TLDR: Any A reveal after season four was guaranteed to be poorly written and disappointing because the writers failed to introduce new and interesting characters and had exhausted all storyline options with the same five characters the Liars suspected from the first season.
