On Buffy:
A three-episode-arc for Gwendolyn Post would have been fantastic, to give Giles a credible watcher nemesis (Wesley never was) as well as to flesh out her connection with Faith and make her betrayal hurt even more.
Olivia. She didn't make too much of an impression in her very few appearances, BUT Giles's love life kind of stagnated and I wish Giles would have had more of a romantic life after Jenny's death. Thus, would have been nice if Olivia had been around a bit longer.
Jesse and Kendra. I'm okay with the amount of screen time they got, but it would have been fun if they had turned up later as the First Evil.
Professor Walsh. For sure, if she had been around for Adam, it would have made the storyline far more interesting.
I thought Scott Hope was cute, he could have been in a few more episodes too.
I know, people will hate me for this, but sometimes I think it would have been fun to see a little bit more of Dawn and her own Scooby Gang, Kit and Carlos, in season 7. They didn't need to have a spotlight episode or anything, but just a cameo to fill out Dawn's world and acknowledge that she has a life on her own would have been nice.
On Angel:
I am always for more of Gwen and Kate, I thought they were fantastic female characters and it would have been fun to keep them around.
Anne also could have recurred more frequently, she kind of just disappeared after her appearances in season two.
As an one-off villain, I really liked the villain in Just Rewards and thought he would have made a great nemesis for a multi-episode arc.