this is my own opinion don't jump me for it.
i've seen a lot of people talk about how romantic it'd be if Mike just suddenly kissed Will after confessing.
i think the most romantic version of that scene is the exact opposite. i don't think Mike would assume. i think he'd ask.
not because he's insecure. not because he doubts that Will likes him. but because he respects him.
Will has spent almost his entire life having choices taken away from him. he didn't choose to be taken into the Upside Down. he didn't choose to be possessed. he didn't choose to be connected to Vecna. he didn't choose the fear, the isolation, the expectations, or the years of surviving things that no child should have had to survive.
so much of Will's story is about things happening to him.
i don't think Mike would ever want to become another one of those things.
i imagine him stopping himself halfway through confessing, looking at Will properly, and actually asking if Will chooses Mike and feels the same way. and then, after Will says yes... "Can I kiss you?"
Mike would be recognizing that so much has been taken from Will over the years that he wants this moment to be one where Will has complete agency. where the answer is entirely his. where nothing is assumed, expected, or taken.
to me, that's infinitely more romantic than a surprise kiss, because Mike has always tried to protect the people he loves, and in that moment, the thing he'd be protecting isn't Will from monsters.
it'd be Will's ability to choose.
after everything he's been through, i think that's one of the greatest acts of love Mike could offer him.
this was all inspired by a post by @wriwi (who also happens to be rewriting several moments in the script which is awesome)