"I have theories … but I don't have any idea yet who is going to be the ultimate villain of this show," Allen, who plays the character Toby Cavanaugh, told ABC News. The show aired its season 6 finale last night.
However, with rumors that season 7 may be the show's last, Allen has his own ideal ending for the series and his character.
"I'm a hopeless romantic. I would like to think of it as 'The Notebook' scenario. He builds this house, and he's growing old. And Spencer comes back, and they live happily ever after," Allen said. "And I would like the show to end with some sort of mystery."
Allen said he's definitely a member of Team "Spoby," the nickname fans have affectionately given his character's relationship with the character Spencer Hastings, and called it a privilege to portray their love story.
"It's a relationship that I would like to have in my real life," Allen said. "[Toby] has a type, this alpha-female girl. And I can understand that. In real life, I find myself very much attracted to women pursuing a passion, and they don't need anybody, a man."
And when he's not working on "Pretty Little Liars," Allen is busy pursuing very different roles in film.
"I play a real-life character who is currently still alive and serving life in prison," Allen said of the role. "Anything that has to do with real-life murder is very scary for me and challenging, and you have to step into these shoes of people who would risk anything."
Allen said the film put him into a position where he had to become an exhibitionist, and he joked that he was naked for pretty much the entirety of the film.
"It was interesting to step into the world of the adult film industry and understand how insanely deep it goes," he added.
Allen is also working on his second book, a photographic journal he's been putting together for the past two years, and recording music. He also wants to illustrate and one day work on Broadway.
With all the interests he's pursuing, Allen has little time for sleep and instead employs a method he learned from his friend Franco.
"A lot of these very, very talented, creative people in the past -- like Picasso -- they would go to bed for three hours and then work for three hours. I try to work like that," Allen said.
Nonetheless, Allen said he's incredibly grateful for all these opportunities and the "Pretty Little Liars" fans who've supported him in all his endeavors.
"The show wouldn't be anything without our viewership, and I really, really adore our viewership," Allen said. "It's amazing that the show has had such a positive and inspirational impact on people."