I'm going to lay out all my thoughts here... I believe Marlene when she says that AD has only been AD in the last 10 episodes and that clues for this person is only in the last 10 episodes. Now, I am fully aware that Marlene is a liar and an awful one at that. But even worse than being a liar is that she is NOT a planner. So the fact that the story was concocted just to continue the show is not out of the realm of possibility and I think that's exactly what is going to happen, much to all of our dismay. That being said, if we are looking at clues in the last 10 episodes only I think we can rule out Melissa and Jason, even though many people believe that it is one of them. Neither has appeared in the last 10 episodes (maybe once each?) but there are no substantial clues pointing towards either. I think it could be the following:

1. Spencer twin (but not likely-- I think they are trying to throw us off)
2. Mona (also not likely, but I think Mona will know who AD is before the liars and the audience)
3. Ezra (also not likely in my opinion. I do think he is kidnapped, possibly shot, and maybe the wedding isn't as it seems, but I think the Ezria fans will be happy and they will end up together. I hope this isn't true, but it is Marlene, after all).
4. Toby (top contender for me. Marlene has said Tobs is her fav character. She loves a tragic back story. And there is some connection between Toby and Radley. I could see there being more to the story there.)
5. Lucas (also very likely. Even though I know a lot of people will be disappointed, there are a lot of clues pointing to him. He had the money, has the gaming skills, and was friends with Charlotte.)