Ali's been Red Coat before, so there's a chance she can be reprising her role.
There's still so much we don't know about Ali's disappearance. It's years of unexplained time! We don't know how involved with "A" she was, and we don't know what shady Red Coat deeds from before are hers.
Despite all the evidence that Ali has changed and Ali is good, we're still not so sure. Maybe she's trying to get close to Charles, because she seems to be pretty sympathetic toward him.
Why She's Not Red Coat:
Ali seems so desperate to get in touch with "A" and Charles, and yet, Red Coat is already in the circle. We have a feeling Red Coat has a higher level of clearance than her.
Ali really seems to have turned a new leaf. We doubt she'd be conspiring with "A" to execute one last takedown.
Mr. DiLaurentis has been obsessively keeping an eye on his daughter. How could Ali sneak out and assume Red Coat's identity when she's under such intense surveillance?