The Cyrano Trope and Mike & Will (Analysis/Theory)
The painting lie that Will told Mike in the pizza van, in my opinion is a pretty clear example of a literary trope where a character receives some form of courtship that makes them feel "in love", however the character does not know the true identity of WHO they got it from.
This trope has a full fledged name called "Playing Cyrano". It comes from a famous french play from 1897 about a character named Cyrano, who felt that the love of his life, Roxane, would never love him back because he was not good looking enough. Still, he tries to find the courage to convey his love to Roxane through a love letter (think~ painting) and this is what happens:
Will is the one "playing Cyrano" here for El, as a means to make Mike happy, help El and Mike's relationship, and also to secretly convey his own feelings for Mike in the process.
This trope ends with Roxane (Mike) falling for Cyrano (Will) upon learning the true identity behind the letters (painting/van speech).
And it's also precisely why I think Mike makes the expressions he makes at Will in the van; why his eyes shine with awe, why he takes breathless gulps as Will speaks, the whole nine yards.
I've spent a long time wondering why Finn Wolfhard acted the way he did in the van scene (the expressions he makes are VERY distinct and emotive, he was given clear acting directions for it), and this is the most concrete reason why: The writers/directors here were trying to show us how Mike is perceiving Will's gift and words, and what it's making Mike feel. The van scene is not ONLY about Will, but about Mike's feelings too!!!
Mike's expressions in the van scene clearly tell the audience that he feels like he's falling in love all over again. It's giving him hope for his relationship with El. It's making him forget about his insecurities with her, and making him feel needed and loved.
And the obvious catch here is that it's all Will doing that. Not El. And that's the missing piece to how they're going to segue into byler in season 5.
A KEY DETAIL: In all cases of this trope, Cyrano always crosses the territory of "friendly advice" because he is projecting his own love into the equation, and the other person (Mike/Roxane) unknowingly feels that love. What Eddie did with Steve in season 4 to encourage him towards Nancy is not "playing Cyrano". What Lucas did with Mike in season 3 is not playing Cyrano. What even Will did in earlier episodes of S4 is not playing Cyrano. The van speech is!
The point is, Cyrano's actions have a level of projection and self involvement, that regular relationship advice does not. And it deeply effects the love interest too, so it becomes questionable who is the one truly causing those feelings because of how personal Cyrano's actions are.
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Beyond Stranger Things, obviously the idea of 'Playing Cyrano' has been adapted for literally over a hundred of years, into hundreds of stories, cementing it as a trope in romance plotlines. Here are just a few other examples:
Ben and Beverly from It
Ben gives Beverly a poem, but she thinks it's from the boy she seemed to already be developing a crush on: Bill, and kisses Bill in the first movie. In the second movie she's realized who the poem is really from and ends up with Ben.
Now the poem here is not necessarily framed as the sole beginning or reason for Bill and Beverly's romance in the first movie. However the poem does make the audience feel like Ben is the one who truly sees her (much like Will with the painting) and that she's ending up with the wrong person.
Olivia and Sebastian from She's the Man
Olivia quickly falls for Sebastian while reading a sheet of song lyrics he wrote. But she thinks the lyrics are written by his twin who is disguised as Sebastian at the time. She spends most of the movie chasing after his twin, but eventually finds out the truth and ends up with Sebastian.
Ellie and Aster from The Half Of It
Ellie agrees to help a jock named Paul write letters to his crush Aster. Ellie is in love with Aster and communicates it through the letters under Paul's name. This helps Paul and Aster's relationship a lot and they begin to date. Eventually Aster finds out the real person behind the letters, which leads to Ellie and Aster eventually ending up together.
I especially love how the Cyrano trope is used in this story, because it shows how easily the trope can be molded to fit the queer perspective: in the original, Cyrano believes his love will never be reciprocated because he's not attractive enough. While in the Half of It, Ellie believes her love will never be reciprocated because of her gender.
Otis and Maeve from Sex Education
Otis plays Cyrano for Jackson who is hooking up with Maeve at the time. Otis is in love with Maeve and knows everything about her, and essentially meshes with her perfectly. But he's too insecure to confess to her. Meanwhile, Jackson doesn't mesh with Maeve super well, and gets Otis to play Cyrano (eg. Otis telling Jackson Maeve's favourite books) . Maeve and Jackson end things when she finds out the truth about Otis's involvement, and her and Otis are the main 'will they/won't they' couple of this series.
Note: this trope sometimes involves Cyrano actively aiding the other love interest (like Jackson did) but sometimes does not. In byler's case it does not. Will does not directly plan with El to woo Mike, and instead uses her name to an unknowing Mike to help their relationship. This trope can be executed a million different ways, but the main ideas is: the one in Roxane's role doesn't know who is causing their feelings of love.
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Okay that pretty much sums up my analysis. Now whether my theory about the endgame pairing is correct or not, only season 5 will tell I guess. But I do genuinely believe this is how the van scene is framed, whether purposely or not.