Qual seria o oposto de um MacGuffin?

Alfred Hitchcock chamaria isso de "suspense" e eu concordaria com ele.

Veja a definição Wikipedia :

Suspense is a feeling of pleasurable fascination and excitement mixed with apprehension, tension, and anxiety developed from an unpredictable, mysterious, and rousing source of entertainment. The term most often refers to an audience's perceptions in a dramatic work. Suspense is not exclusive to fiction. It may operate whenever there is a perceived suspended drama or a chain of cause is left in doubt, with tension being a primary emotion felt as part of the situation.

In the kind of suspense described by film director Alfred Hitchcock, an audience experiences suspense when they expect something bad to happen and have (or believe they have) a superior perspective on events in the drama's hierarchy of knowledge, yet they are powerless to intervene to prevent it from happening. Films having a lot of suspense belong in the thriller genre.

"Alfred Hitchcock: a diferença entre mistério e suspense"

Do vídeo:

Mystery is an intellectual process, like in a "who-done-it". But suspense is essentially an emotional process. Therefore, you can only get the suspense element going by giving the audience information.

E, na diferença entre suspense e surpresa:

There is a distinct difference between "suspense" and "surprise," and yet many pictures continually confuse the two. I'll explain what I mean.

We are now having a very innocent little chat. Let's suppose that there is a bomb underneath this table between us. Nothing happens, and then all of a sudden, "Boom!" There is an explosion. The public is surprised, but prior to this surprise, it has seen an absolutely ordinary scene, of no special consequence. Now, let us take a suspense situation. The bomb is underneath the table and the public knows it, probably because they have seen the anarchist place it there. The public is aware the bomb is going to explode at one o'clock and there is a clock in the decor. The public can see that it is a quarter to one. In these conditions, the same innocuous conversation becomes fascinating because the public is participating in the scene. The audience is longing to warn the characters on the screen: "You shouldn't be talking about such trivial matters. There is a bomb beneath you and it is about to explode!" [emphasis added]

In the first case we have given the public fifteen seconds of surprise at the moment of the explosion. In the second we have provided them with fifteen minutes of suspense. The conclusion is that whenever possible the public must be informed. Except when the surprise is a twist, that is, when the unexpected ending is, in itself, the highlight of the story.

Esse senso constante de "como eles vão lidar com essa situação que eles nem sequer sabem que existe" ... isso é suspense.

Temos um casal, Jesse e Drew.

• Jesse trai Drew, mas Drew não tem ideia.
• O público está ciente da trapaça porque ocorreu no filme.
• O público fica em suspenso imaginando se Drew descobrirá e qual será a reação.

Nós, como público, experimentamos esse "sentimento de fascinação e empolgação prazerosos misturado com apreensão, tensão e ansiedade ", esperando para descobrir se Drew jamais conhecerá a verdade.

Acho que você pode estar procurando Ironia dramática :

Irony that is inherent in speeches or a situation of a drama and is understood by the audience but not grasped by the characters in the play.

Eu acredito que você esteja descrevendo o POV Onisciente:

‘Omniscient POV’ in film means: a point of view outside any of the story’s characters. The audience knows and sees everything that is relevant to know about everybody in the story

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Eu não sei se eu chamaria isso de oposto a de um MacGuffin.

No roteiro, a situação ideal é que sempre, qualquer coisa mostrada na tela tem que ser significativa para a história ou os personagens. Se não for, então pode simplesmente ser apagado. No caso do MacGuffin, como um dispositivo, é importante no começo, mas enquanto a história evolui, perde sua relevância.

Nesse sentido, talvez o dispositivo oposto do MacGuffin possa ser o Gun Chekhov :

"The Chekhov Gun is actually reference to a literary technique known as Chekhov's gun, whereby an element is introduced early in the story, but its significance does not become clear until later in the narrative. The concept is named after Russian playwright Anton Chekhov, who mentioned several variants of the concept in letters."

Portanto, o MacGuffin é importante em um estágio inicial.
A Arma Chekhov tem importância tardia para a história.
Ambos dão aos personagens um novo impulso, mas em momentos diferentes: cedo e tarde na história.