Okay, well, let's think of the perks of Ashley's character! She's a bit more "fun-loving" and social from what I can see. She enjoys a good fight and takes pride in her Spectre status come ME3. People call her racist, but she does grow attached to Tali and possibly the other alien characters as well (even going as far as to say that she saw Tali like a little sister) which suggests a change in her attitude towards the aliens, or at least the ones who are good.
Her distrust of aliens stemmed from their rocky history with the Alliance during the First Contact War, and Shepard is the only Alliance commander to bring aliens onto their ship. She's extremely loyal to the Alliance, and likewise, her personal feelings are overruled by her loyalty, so she will follow you either way in ME1. If your character is also an Alliance loyalist, then that could be seen as a very positive trait. It can help your Shepard to understand her better in ME2 if they resent being with Cerberus just as much as she does.
So is your Paragon just as loyal and dutiful? Did they choose to uphold their responsibility as a Spectre to preserve all galactic life, but still understand Ashley's inhibitions when it comes to aliens working on their ship? If so, then a close relationship is very feasible. You could still headcanon a growing understanding from Ashley once she becomes a Spectre. She'll always uphold her duty and protect the council during the Cerberus coup regardless of her personal distrust for aliens.
Another thing, she attempts to appear tough in front of her comrades, but is evidently flustered when it comes to a male Shepard. Which is kinda cute, if you're okay with the occasionally self-conscious type. You get to tease her a lot. :D
No one can deny that for Paragon, the personality with the least resistance is Kaidan. But being a Paragon literally means you can compliment any of the romance options as long as you support them. All of the relationships are worth it and have their perks. Except for Jacob, in my opinion. I didn't get anything from him before I even knew what happened in ME3...