What are some things they could plausibly do in the second season that would ruin the show?

...

Focus more on cgi monsters.

They're not that dumb. Implausible, I say.

It's possible they could lose track of the balance between suspense and pathos that worked so well for them.

If they go too much 'alien' and not enough 'goonies' or 'stand by me' they'll be fucked.

The first season wasn't very good.

[Spoiler]The scene where the teacher explains parallel dimensions with a flea and an acrobat was retarded [/spoiler]

Are you fucking serious???

The sheriff sneaking around a government facility by sucker punching everyone in the face was even more stupid.

So was the scene where the girl decided to leave behind her stuff crawl into a bloody tree hole. What did she think was going to happen?

Turn it into a sitcom with a laugh track but the laugh track is really scraping noises and heavy machinery played a little too long so it covers up what the character says after the joke and makes it really hard to follow

You guys understand that movies and TV aren't real... right?

Sometimes they follow or reference an existing set of conventions and tropes that we call the language of film.

I don't even see where they could take it from here in a compelling way. The appeal of the first season was seeing people come to grips with a kid's disappearance and other weird shit in distinct, but ultimately connected, ways. The Upside Down itself isn't really that interesting. I'm not sure how Stranger Things II: But More! could carry that kind of fun suspense, but who knows.