Buffy:
S7- Reduce the number of potentials so that we can get to know them beyond very superficial levels or give up on the idea and remove them completely. I wish I could change even more than that tbh, but you said just one thing so I'll leave it at that. To make a long story short, I want more scoobies time and I want a tighter, slimmer, streamlined cast of characters. I would almost complete redo the whole season though.
S6-I am totally in support of a dark and depressing season (and if it had been done masterfully it would have been my favorite season), but I would rethink the nature of their various dark paths and thoughts and feelings. Have them come at it from more interesting and profound places. My problem with a lot of the characters direction and dark places and feelings was that it all seemed too mundane, pedestrian, cliche, uninteresting. I have had many dark periods, and thinking back my version was way more interesting than the Buffy S6 characters if I may say so. It almost gave me the impression that the writers had no real truly dark experiences of their own to draw from...or their versions were also mundane, pedestrian, cliche, etc. It could have been so much more than it was. I love dark stuff and dark places, especially as a one season break (a one season moment of serious introspection) for the show.
S5- I like the idea of the knights of byzantium, but I thought it was poorly implemented. I would rethink this and come at it from a different angle. Flesh them out more, develop them more, maybe introduce them just a little bit earlier somehow and make their existence in the Buffyverse a little more realistic. But I loved the idea of them, so I would keep them, but adjust things and make the idea better. Other than that, Season 5 is perfect to me.
S4- I struggled with the criticisms of the initiative plot line on the boards for a long time. I wanted to like season 4 badly, and I still do like it, but I have to concede that the initiative arc was not so good. Throw it out and start with something else. But by doing that I would keep Restless EXACTLY the same at the end, and I would like the core scoobies to invoke the primal power of the slayer line in that spell again to make Restless as an episode possible, which means that the big bad has to be dangerous enough to warrant such a power move.
S3-Develop the whole "fallen Faith arc" thing. I'm in the minority here, but I wasn't completely satisfied or convinced by it. I know what the writers were intending but I think they fell a little short in the execution. I demand a little more development of this idea. Other than that, S3 is the other season I'm very happy with as is.
S2-Improve the monster of the week episodes. I'm warming up to some of them (some aren't as bad during multiple rewatches), but some of them are irredeemable efforts.
S1-Make the season a full 22 episodes. I guess I'm not removing anything with this answer either though, so I would remove the annointed one idea, remove 1 monster of the week episode and replace it with an episode focused on the large arc with more development of the master.
S5 and S3 don't need too much changing by comparison to the other seasons. They are solid A efforts.
Angel
I love S5, S4, S3 a hell of a lot. I'd keep them more or less the same actually. Tweaks here and there, maybe take out the Jasmine thing at the end of S4, but all in all, I'm happy with them.
S1 and S2 were good, but not great. I can't really pin-point anything really. There wasn't any one or two glaring things that brought those seasons down for me, they were just...I don't know, adequate for me, all in all. I'm in the minority again, but I personally didn't get really excited about Angel until S3, and then it took off. I guess it was the smaller stakes if I had to really pin it down to one thing. It seemed so small time, such small potatoes when compared to the parent show. I liked the Darla/Druscilla stuff, I liked the main cast, some individual episodes were memorable, I liked a lot, but my final impression of the first two seasons on the whole was that they were just so....small. When the stakes got raised dramatically the show got a hell of a lot more interesting to me.