I'm not sure that "Lash" could regrow into a full angel since she was never actually an angel, only the shadow of one. I don't think she could grow into something more than what she once was.
I agree that Lash couldn't become an angel again, but mostly because it doesn't seem to fit her and the way the Dresdenverse works - you can change and grow, but you can't ever go back to what you were before, which would be what Lash becoming an angel would be.
That said, she can, and I think will, come back as something more than just a shadow - Harry gave her a Name, and names are Important in the Dresdenverse. Remember how Ivy reacted when he named her? And that tidbit about Bob only really having personality because Harry named him, making him something more than before? And in Dead Beat, it was Harry's mentioning of the fact that he gave Bob a name that gave him the free will to act. With all of this evidence, it's fairly clear to me that his giving Lash a name has already inherently made her more than she was before, and it would just make sense for that to mean she could become something more than just a shadow in Harry's mind.