Working backwards...and potentially full of spoilers for the uninitiated.
Buffy, Season 7:
Introduce Wood as a good guy early. I don't think we profited by keeping his allegiances a mystery.
I think someone else on here mentioned tightening the lineup of potentials and expanding on their stories so we actually care about them. I agree.
Get Spike out of the basement of the school earlier. So many episodes were spent wondering about Spike. I feel the chip, trigger, and insanity plot could have overlapped in some places, been advanced sooner, and have allowed him to become an asset to Buffy earlier. Plus this could have allowed for some of the more interesting aspects of their relationship to be developed, instead of being crammed into a couple of episodes at the end.
Rework some of the threatening scenes to actually be threatening. Some of the conflict lacked so much strategy it was hard to believe there was a threat at all.
Buffy, Season 6:
(Sorry, folks. I loved Season 6, and it's one of my favorites. Wouldn't change a thing).
Buffy, Season 5:
(I also love season 5, but for the sake of saving Season 4, I'd gift the detoxing Riley to that season, and instead hammer on his insecurities alone.)
Buffy, Season 4:
I wish the mad scientist and the monster plot could have been continued to the end with Maggie Walsh. I realize this was the original intent, but the actress became unavailable partway through the season. It would have been better.
Plus, detoxing Riley should have been in this season, so we get a clean slate to focus solely on his relationship with Buffy in season 5.
I also absolutely hated the cutesie plots with demon roommates, magical booze, and vampiric queen B's. I realize these were the early staple of the show, but they aren't the plots that stick out to me, and their themes are too much on the nose for my taste.
I did like the garbage boyfriend, Parker in this season, Harmony's reintroduction, and the development with Riley. I'd replace the garbage themes with those characters.
Season 3:
Gotta love me some Faith, and that whacky Mayor. I really love this season as well. I probably wouldn't have involved everyone in the school for the final battle, though, however symbolic.
I also hated the LA plot in "Anne". I probably would have used her time away from Sunnydale differently. She could have found herself in other ways, maybe even met up with Dru and Spike again. As you'll see later, though, I dislike alternate dimensions as a rule, though. I also hate everyone's behavior on her return. That could have been reworked.
Season 2:
I hate the Acathla and the Judge storylines, and think Spike, Dru, and Angelus could have had some better threat looming over Buffy, like a personal one, maybe her friends and family being targeted. That could have been expanded a bit more.
They did this briefly with Joyce and Angelus, but I feel it should have gone on longer. This whole season could have been like really good horror viewing just from this threat alone. Not only would it have been more menacing, but it would have made more sense. Angelus likes comfort. Why the hell would he destroy the world? I do like Spike's alliance with Buffy, though, but he's got enough of a reason to double cross Angelus without the whole world being in peril. Really, Buffy's whole world is the only one that needs to be in peril.
Use Kendra and Dru more, not necessarily together, just more of both of them.
Get rid of Whistler. I know, like Prof. Walsh, there were plans that got foiled, but in the end, it didn't work out. Angel doesn't need a guide. He could have found Buffy on his own and figured everything out from the shadows.
Season 1:
Borrow some of the Darla backstory from later Angel and gift it to this season.
Also, don't bother with Jesse, as he was both too much and not enough.
Put more Darla in this season, and don't kill her off. Have her just cowardly run away after the Master dies.
Rework the stupid hyena storyline. I get the metaphor, but it was both gross and just about made Xander intolerable. Maybe take the girls out of the mix, and don't add cannibalism to the story.
Introduce Olivia in this season and start building her up so we actually care when she shows up later.
Take out any plotline where students are suddenly geniuses at making magical or superhuman scientific things happen. This goes for all seasons. I'll believe the evil surrounding the Hellmouth can make extraordinary things happen. I'll even buy witchcraft as an explanation, but I draw the line at teens randomly becoming evil geniuses. Later, Warren I get, but he should be the exception, not the rule.
As for Angel the show:
I liked the show best when he was a vampiric Batman. I hate time travel plots and higher powers incarnate. Those things are fine for Dr. Who, but I don't think they had a place in the Buffyverse. I also hate Wolfram & Hart as a plot device, purely because I hate the idea of an omnipotent evil potentially controlling everything but doing nothing about it, and I hated the pacts everyone had to make in that storyline.
Also, the alternate dimensions got whacky on this show, whereas I bought them on Buffy. I'd either totally kill the Connor story or keep him young and make it better. I also liked Darla evil, so maybe Connor comes from someplace else. I don't know.
It's not my favorite show, though I do like some characters and episodes. Lorne was great. I'd probably bring about Wesley and Fred sooner. Gunn was a good character with some good plot points, but the lawyer thing kind of ruined him. I also liked Cordelia on this show, but I dislike the level of power she got. They should have stopped with intolerable visions. Being super badass is Buffy's thing, and having massive magic mojo is Willow's. Being burdened with crushing visions, though, works well for a character who used to be a bully.
