Just some things that won't make sense unless Byler is endgame:
The French VA for Mike said it was Will's words/love that led Mike to say he loves El (a portion of a livestream that was later deleted). Will's words getting Mike to say "I love you" to El needs to be addressed
Noah saying in October 2024 (after reading the finale script) that you could never really tell if Mike and Will had a really special friendship or if it was something romantic, and him proceeding to tease Byler throughout the press run
The Duffers telling Finn that Mike's cluelessness in the van scene would "pay off in the end"
The Cyrano trope with the painting
The snowball to an avalanche conversation needs to pay off
Signals = brush of the knee, bump of the elbow, shared look (all things Mike and Will do to each other)
The dick jokes and pipe scene with Mike and Will (throwing in homoerotic jokes of Will with his supposedly "straight" bestie)
Mike constantly being called the "heart" of the group and Will the "sorcerer" (names they gave each other)
Both their finale colors being green (Will wearing a green shirt, Mike wearing a green vest/hat) instead of their standard blue/yellow
Their costumes being complementary throughout the seasons
Mike getting a triangle put onto his shirt when the costume designer said she put triangles on Robin's clothes as a subtle gay symbol
Mike's one-way sign and closet imagery and his closet being destroyed this season by his homophobic father
Mike constantly being associated with rainbows including in Season 5 Vol 1
None of Mike's behavior in S3/S4/S5 Vol 1 make sense from a lens of him being straight
Finn saying Mike was trying to be "as normal as possible" in S4 and "maybe he's into some new things" and we're not shown anything new he's into...
"WE'RE FRIENDS! WE'RE FRIENDS" defensive much??
"It's not my fault you don't like girls" but Finn said Mike (obv before S5 Vol 2) didn't know Will was gay, and we know Mike isn't homophobic, so what else was this besides projection/internalized homophobia?
Literally all of Season 4's Byler content
Mike and Will being paired together at the end of Season 4 between two couples
All the canon couple parallels with Byler
These are just some of the things that came to mind. There are far more.