Since this upcoming 'sequel' is on debate and now on production, I just wanna inform you something. This is my personal theory, so mayhaps it isn't possibly correct in a certain way.
As mentioned in the title, this sequel is possibly an origin story on how monsters has being made. Yes, in other words, prequel. You can look at this continuation webtoon before Netflix made any confirmed statements.
Shotgun Boy
As you can see, Song Kang is in consideration whether he should join or not for this project. Unfortunately, if he joins this continuation, mayhaps he has the least screen, somehow in either the last episode or in between, because to tell the truth, so far from the given webtoon, there's no character of him; hence replacing ML aka the origin innocent-monster we read (I think there is one from the drama, I cant recount but it's in 2nd last or last ep)
I've somehow heard the rumor (now the article deleted) that Jinyoung B1A4 was considered to be as ML. Mayhaps he's the only protagonist on the sequel for now (or maybe replaced? Who knows?). And yes, speaking of justice, Lee Do Hyun should reprise on this sequel, because he, too involved in this webtoon.
As of this date, the prequel webtoon is reaching the end of the story. Maybe 2 months or so? We'll see. Maybe they'll release this sequel earlier before releasing the last episode of the webtoon (patterns like previous works). So, calm your spirits down before the storm rides out.
While wait, I suggest to read. Or not, your choice.