I wouldn't change too much as I love the series as a whole but because the earlier seasons are my favorites.....
Go dark, but try to maintain the balance of lightheartedness/family with the dark which Angel as a show was able to do. The characters went to dark places but the show's tone was still frequently uplifting and not so depressing.
More male characters, get somebody, Xander seemed so lonely without Giles or any male to talk/relate to. In the early years we had Angel, Giles, Spike, Oz, Xander. Then we have Xander and Spike who doesn't really fit into the Scooby gang in the traditional sense.
Lastly, better villains. The early seasons had consistently threatening awesome villains. The Master, Drusilla, Spike, Angelus, The Mayor, Faith. Adam, (and many fans argue Glory) just don't stack up. I am glad they brought the First back and I remember hoping before Season 7 was even made that they would make the First the final villain. Still I think the villains could have been more awesome in the later Seasons or even take a cue from Angel and have one villain that spans several seasons.
Although I enjoy the later seasons and prefer the earlier seasons of Buffy to any show on TV ever, I think the Angel writers handled all the young adult issues and metaphores than Buffy did after high school with more hope and consistency.