I think this is a really great point. After the incredible character development Tara had over the sixth season, and the impact her death had on Willow, bringing her back, as the first person The...uh...First (ah, puns) poses as would have been a hell of a way to introduce the new big villain. The scene with First-Cassie was remarkable, really haunting, but having it be Tara, the ultimate nice character we'd grown to trust and rely on, would have been something else entirely.
Now, that being said, I don't mind that it didn't happen either. I'm not entirely sure why, storyline wise, The First couldn't pose as Tara (if there even is a reason), but for whatever reason it always added a degree of loss and longing for Willow because The First couldn't do it.
I'm not entirely sure the two parties couldn't make it happen. I've seen commentary episodes where Joss has stated that he had designed an episode where the real Tara would return after Buffy is granted one wish, for anything. She deliberates with Angel about making him human again before the two of them instead decide to bring Tara back to life (which would have been a classic Angel move). I'm not sure how they would have worked around a repeat of bringing someone back from heaven (I don't think there's anyway Tara isn't in heaven), but I would have liked to see that. I loved the fallout from Tara's death, Willow in S7 and I liked Kennedy, but that would have been a special moment.