Was that the Series Finale?

the american is most likely Hopper

I'm guessing at least one more season. I hope they can keep it awesome. I'm afraid they are going to ride it too far. IMO, even this season they spent a LOT of time on relationships and it felt like filler. Like, we need 8 episodes, so we'll spend a ton of time on relationships of characters...which was fine in the first episode but it started to wear on me a little...Also, Hopper was written very differently this season...I didn't hate it, but it was a little jarring...as was Steve. Oh well, I'm looking forward to what comes next...

I'll be disappointed if Hopper is alive and well if and when season 4 premieres, since Tom the newspaper editor commented "Fact, in the real world there are consequences to your actions" which should be strong foreshadowing that Hopper won't appear in season 4 but if there's a season 5 then it's fair game he's still alive

There are a couple reasons Hopper is alive. Obviously the old "main characters aren't dead if they don't show you their death". Same reason why Brenner is alive and kicking somewhere, possibly working with the Russians?
Although he was more about the kids than the "gate".

The obvious clue is saying an American is in the cell when the Russians called him that all season. But to me a more subtle possibility is that L can't use her powers. I know why that was important for the season, but to me an important reason to have her not find them at the end is to keep her from mentally finding Hopper.

Finally, the writers had to know people would consider that he jumped into the gate. So if you want his death to be emotional and you don't want to lead people on, you show him dead. If you are killing off the most popular character, you get your money's worth. You don't leave it open-ended. You make it obvious so the emotion is full and unquestioned. So perhaps he jumped into the gate, and the season is used showing how he ended up in Russia. Perhaps explaining where the Russians got the demogorgon. Perhaps at some point in the season L gets her powers back and finds Hopper in the prison.

Also read that the Duffer Brothers thought this would be 4 seasons, 5 at the absolute most. And that's them saying this recently. I don't think they thought they would even get this far. Saying roughly "we can't really have them getting into trouble every year".

The Duffer Bros shouldn't have added the Tom Holloway line, because now they're being creatively disingenuous if they ignore the "Real consequences " element.

This is what I thought too.

I really liked Season 3, but am struggling with a loose end:
- Why did the rats, then hosts, obsess with fertilizer/chemicals? This was never answered. Did it help break down the hosts into fleshy, gooey messes that were absorbed into the megagorgon?

yes

I was just talking to my brother about "the American" at the end. He thinks it might be Murray. He said he didn't notice Murray with the rest of the crew in the end scene.