Yep, DWH1982 - hard times 
It's too much of work to go through all possible headcanons in my case, therefore I'll pick the one which is used in my project right now. Besides the starting point - Ashley gets rescued on Eden Prime by Shepard and Kaidan in 2183 - a lot of stuff changed. Shepard doesn't die, for instance, but gets wounded enough for a long recovery time. He also works for Cerberus - as a double agent. However, to get things done and to end with Cerberus, he had to cut all ties to the Alliance - including Ashley. And, to ensure things work out like planned, Shepard is not allowed to tell her anything.
Pretty much that - at least at current planning stage. Maybe she DOES know but starts to believe Shepard defected - however, it's the basis of a lot of distrust between both. All I did so far is to remove the "Shepard-died-part".
Ashley's part in here is: I like the way she was in ME1. I'd love to explore her family side a bit more, especially her relationship to her father and grandfather. I believe, her grandfather is still alive in ME1, her father is dead 'though, so there's some story to be told.
For that, by the way, I do virtual interviews with most of my characters - to get in touch with them. That's also a trick a lot of professional writers use. I plan to do it at least twice - one at the beginning of ME1 and one at the end of ME3, to "simulate" character growth.
Just an idea, how it works (this one is post-ME3):
CptData: So you had a pretty rocky relationship to your grandfather?
Ashley: Depends. I disliked the way how the Alliance treated him after the war - after all, he tried to save thousands of lives. Basically, he got kicked upstairs, to some sort of a desk job. When he quit, I thought he simply dropped the ball instead fighting for it. When I joined the Alliance, he argued against me - and when I found out he was one of the founders of Cerberus - I haven't talked to him for years.
CptData: I see. So, the hatred for Cerberus is based on what your grandfather did?
Ashley: No, it's not like that. I was upset because my grandfather was involved founding Cerberus. Especially after what I've learned just too soon after joining Shepard's crew. Abducting humans, illegal genetic research, Subject Zero, Operation Overlord - all that. When you have to go through all that, learn all that, you really hate everyone and everything involved with Cerberus. That also included my grandfather, although I know he had a different agenda than Jack Harper.
CptData: Different goals?
Ashley: He wanted to protect Earth, the colonies - the Alliance itself. I'm sure my grandfather couldn't forgive himself what did happen on Shanxi. Not trusting aliens is one thing. Using them and own people for experiments like those on New Horizon - no, that's something my grandfather surely didn't want. But Jack Harper did. That was his doing. Doesn't change my oppinion on Cerberus and before learning the truth - well, my grandfather didn't deserve the thoughts I had.
CptData: What did change your mind?
Ashley: My grandfather gave me several datapads, just mere weeks before he died. First I didn't want to read any of them, fearing all I knew about my grandfather would be true.I found out that wasn't true - instead, I feared the idea all I knew about my grandfather wasn't true. And I was right - in the end, I learned all he wanted was protecting mankind.
CptData: Thank you, Ashley. So is there anything else about your grandfather?
Ashley: Yes. I think he would have liked Shepard - and approve what we two have now. Too bad I couldn't tell him when he was alive. But I know he's now at a better place watching us, like my dad.