yea I don't think he will transform into the monster but the slug will

Is Will transforming into the monster?
I thought it was more of an "Alien" reference.

didn't notice he was eating the egg in that scene, makes sense of what he was doing now tho.
but now the question is where did the egg come from?

The egg was a snack for the monster, it's what it was eating in the first scene. The concept artist said it was something the monster ate but Millie confirmed (NYCC) it was what it was eating when she touched it in the first scene we get a visual on the monster.
For me I thought it was just an Alien reference, like our monsters gonna eat your monsters eggs.

It's always possible Joyce and Hopper were too late. The boy they rescued isn't Will but a duplicate that nearly died when they "disconnected" it prematurely. It may have all of Will's memories and so believes itself to
be
him - which in a sense it is. Perhaps his body is producing those slugs one at a time, and any which come in contact with another host will burrow into them, kill them, and grow into an exact copy. This is more
Invasion of the Body Snatchers
than
Alien
.

They haven't copied to much. The slug was original and he didn't die. But the whole host thing and that random egg was reference to the movie. Im excited to See what they will do with it.

I believe the slug part is the same as how the aliens in Alien reproduce. They use a host body that then produces a new alien.
It does seem to be heading that way, but personally I would prefer they deviate a bit from that theme rather than just straight out copying Alien 79 Xenomorph's characteristics.
I would like too see that the slugs keep the monster's food "preserved." Their own version of a refrigerator. The victims are not hosts for new monsters, the slug system keeps the victims fresh - feeding the comatose body oxygen and nutrients, to keep it edible over a longer period of time.
But that doesn't account for the egg, so sadly it looks they are going to straight out copy the Xenomorph's pattern.

One of these days I'm going to cut you into little pieces.

This show has allot of reverences to 80's movies. I believe the slug part is the same as how the aliens in Alien reproduce. They use a host body that then produces a new alien. So I'm thinking the slug will turn into a new monster. Seeing it went down the drain it will probably make the sewer or something its new lair and turn that area into a new upside down.

I think he was used as a host, but he wasn't there long enough for it to kill him, from how the other bodies were that we saw, i believe that they were used as hosts, but as they weren't rescued they died as a result
So i think Will while obviously infested to some degree with the slugs/creatures, i think it's not enough to kill him, he just coughs them up, which isn't exactly ideal and has to have some serious consequences that we will discover in season 2
I hope he doesn't turn evil though, i think it would make more sense that he has the ability to enter the Upside Down and they use that to find and bring El back, who is trapped inside the Upside Down
However, the slugs/creatures he is coughing up, leave many possibilities and whatever they decide to do with that, because they have to lead to something, otherwise it was pointless showing him coughing them up