-Aria & Alison used to switch personalities just like Alison and CeCe
-Aria has a mental disorder, this caused her to become obsessed with Alison’s personality
-A sends texts like Alison would and/or pretending to be Alison.
-So Aria is A, and A stands for Alison, the personality she is obsessed with.
(Side-note: I’m going to keep posting theories, most likely elaborating on this one because I know I can find a lot more to back it up)