My money is on Lucas being AD
Melissa, Wren, and all other characters returning will be red herrings/closure to the series.
The girls will be playing AD's game bored.
Lucas is an game app developer if i remember correctly.
Lucas being AD makes sense because he's always known about A and has been involved.
I keep thinking back to season 5 when he was on the A team with Mona, Melissa and others.
PLUS
The girls are basically living in his condo, which i'm sure he has bugged. Hanna's bridal party incident, wasn't an accident. He's always out of town which gives him the chance to play peeping tom.
Something that always bothered me was how he was so quickly to be Hanna's Ali. I'm curious to why he jumped at the opportunity so quickly. Is he hiding something?
Lucas has all the reasons, opportunities, and motives to be A.