i think id be curious to see them do as many seasons of buffy as possible, all twenty. it may seem like a lot, or it would get boring, but i dont think thats the case. at first, the writers and producers etc. might stuggle to get ideas, but i think it will lead up to something great. many more stories could be told. always. until they all die. because until then, they will have lives, and when there's magic involved, how can their lives not be interesting? look at all the fanfic! they're chock full of usable, remaining ideas (the good ones, anyway 🙂). and if not, think of this. there are some really good shows that get cancelled after only one season because they werent noticed, or hadnt hit their stride. it might be like that. re-starting out, waiting to hit a stride, maybe even changing direction a tad. the big difference is that it would be noticed and criticized, so it would be harder on them to keep their audience impressed and waiting.
thats not a positive theory, but it is a possible theory, and who wouldnt want as much buffy as possible if joss whedon hypothetically started churning 'em out? 🙂
as far as angel, i agree with the premature cancellation opinions. it should have at least two more seasons, to match buffy's number, and get to all those season 6 plots which were in the works.