I've been rewatching S1. And I'm finding some of the stuff used for shitty shit now was seeded as far back as then. Even though I think things were intended to be used differently than they ended up being used.
Jackie was never intended as an actual serious love interest for Marco. She was supposed to be the unreachable high school stacy that he thirsts for but never gets his hands on, and then grows out of chasing to settle for Star.
Meteora/Mrs. Heinous had her club cheeks as far back as her very first episode, but when they manifested, she didn't display any monster traits. Chances are she wasn't intended to be what we now know as Meteora, but instead a past butterfly who fell from grace and then suppressed her butterfly traits, created a new identity and started a finishing school. Most likely, race was never an intended theme with anything involving her in the first place.
Janna was being even more creepy around Marco in particular than she still is in later seasons. She was likely intended to be some sort of rival for Star.
The monsters dindu nuffin subplot was ironically seeded in S1 through Mewnipendence day. But already then, the lacking introspection in the more politically minded side of writing shows in how monsters are implied to be in the right, in spite of admittedly lacking the technological prowess to make so much as a fucking spiked mace or a suit of armor. Let alone a gun. On top of the whole 'you could always just leave us alone' deal they've got going on. Literal cavemen with animal faces who attack you just for it's own sake.