The pattern does not seem to fit.
List of Big Bads
- Season 1 is all about the master. The Annointed One is a tool he uses.
- Season 2 is the exception - it fits clearly
- Season 3 is the Mayor
- Season 4 is the Initiative
- Season 5 is dual - Glory and the Season Close Sacrifice
- Season 6 lacks a singular big bad.
- Season 7 is The First
Season one, the Master is clearly present from the beginning. The Annointed One is not convincingly threatening, but is shown to have some role later. The Annointed One is a tool, not a big-bad. He is, however, a good hook for something to kick off season two if they got it.
Season 2, with the Annointed One, really seems to have been the change of direction. But once the direction changes, it's clear that the Drusilla, Spike and Angel show is the real meat. Whedon has noted that it was a change of plan due to use of a child actor.
There is also some minor misdirection with the Judge. Had the issue been handled differently, the Judge could have been a decent big-bad for the season.
Season 3, the Mayor was previously foreshadowed as a future big bad (in season 2), and his sponsoring of Faith really shows him, not Trick, to be the big bad early on. There are a number of off-season-plotline threats.
Mr. Trick is a clear evil, and may be seen as a mild misdirection for an episode, but when one looks at season 2 in retrospect, it's clear something is up with Snyder being worried about a big bad in his chain of command. That turns out to be the mayor.
If there is a misdirection, it is in Faith not being the big-bad, after the epsiodes 3.1-3.4 mini-arc.
Season 4 foreshadows the Intiative itself as the bigbad, and while Buffy essentially infiltrates the Initiative, the misdirection isn't that the real bigbad is Adam, but that the initiative isn't bad. The whole season leads up to Adam from very early on, mostly with foreshadowing about the room in which Adam is, as Adam shows the real purpose of the Initiative's leadership, and He is the culmination of that. Really, Doctor Walsh and her monster are jointly the big bad, in the same way that Frankenstein and his Adam were in Mary Shelly's novel.
Season 5's big close is forshadowed from the previous season. (What Were the Clues Foreshadowing Dawn Summers Appearing?) The whole mystery of the Key runs throughout. Glory is the means of setting up the real big-bad - the need for a "human" sacrifice, and the resulting self-sacrifice by Buffy. Not so much a diversion, as a cause & effect, much like the link between the Initiative and Adam. We'll see this pattern again in Season 7.
Season 6 lacks a singular big-bad. The Nerd Herd, namely Warren, Andrew, and Jonathon, fill that role, but really, they are second fiddle to the real big-bad of season 6 - Adulthood. This is not just my idea - it's shared by the writers of the RPG, as well.
Life in all its glory and tragedy, the choices it presents
us, and the consequences of bad decisions are the major
themes running through the sixth season. Both heroes
and villains find themselves dealing with life as adults,
and making a hash of it. For some, just living is challenge
enough; for others, tragedies are sought out and
embraced like old friends.
— Carella, et al, BTVS RPG Revised Corebook, p. 25
We also get the whole issue of Moral/Ethical codes cropping up. This is really a Theme season, not a "Big Bad" season.
Still, the season itself ends with the culmination of the effects of the Nerd Herd - as dubbed in the RPG, "Darth Rosenberg" - Dark Willow. She's the effect of the Nerd Herd - well, of Warren going on a rampage when thwarted one too many times, and shooting Tara.
Season 7 really is all about bringing things to a head. The First and the Hellmouth itself are cast as a dual-facet of a singular big-bad: the status quo. It's not a case of misdirection - the First has to be dealt with, and then on to the Hellmouth, but the First charging up the hellmouth is the real threat, and the final episode really isn't the story climax - it's the story denouement, the natural arc fall down result from the First and his actions.
The Season Big Bad isn't obvious from Episode 1 in any season. It always is a slow reveal.
And the season closers aren't always the Big Bad themselves, but in seasons 5, 6, and 7, the results of the actions of the big-bad.