I've finished my full series rewatch, and finally saw Forever Charmed again, since years ago.
Forever Charmed was good and I did feel it was a great ending, but then it felt rushed, I think they should of had 23 episodes, and a 2-hour final, so many things wished we had. Of course Prue back, but even in the time-travel, they couldn't show Prue as a child, no Shannen needed. It would of been some closure.
I actualy like Billie, she wasn't as bad as everyone says she was, and I think she was a great addition, and yes I think the only complaint was it was in the final season, well if she came in a season or two earlier, that could of helped.
I really feel the tone change from season 4 to 5, but I actually think it really changed once Wyatt is born.
If the show had continued with Prue, I think Paige could of been introduced in place of the Billie/Christy arc, and have her going up against her sisters, it would of been personal, been about the charmed ones, and brought closure. Although I'm not sure the show would of have the sisters killing Paige in the series final, may have been too dark, maybe she could of been redeemed like Billie in this instance.