The Stinger revealing the existence of other superpowered beings like Brandon gets cleverly disguised and foreshadowed with the webpage Tori finds in her research of the night Brandon’s ship arrived. It clearly shows multiple orange blobs in the sky, heavily implying that if one of these objects was Brandon’s ship, there were more of them arriving together.
The ending. Brandon Breyer (Brightburn) is free to wreak havoc and destruction around the world. The Stinger implies there are more supervillains in the universe of Brightburn as evidenced by the bald man's speech about an aquatic man and a madwoman with a lasso. Imagine the universe's equivalent of the Justice League (among evil versions of other superheroes and/or superhero teams), but all evil.
Unlike Superman, we don't know if Brandon is the Last of His Kind or if there's others like him sent to different planets with the same purpose, acting like sleeper agents. Given that most of his physiology appears to be human (and it's unlikely the aliens actually are that human-like), the latter case gains much more traction as it means they specifically tailored him to fit in with the dominant species of to the planet they sent him to. The fact that we don't learn why they're doing this makes it all the more terrifying.
Brandon (and presumably the other supers) were sent here in a ship urging him to take the world, suggesting that this was a deliberate invasion plan. This race was presumably aware of how powerful he'd be compared to the native population, otherwise an invasion force of six beings would be laughable. So why send him in a ship made of the one material that could hurt him? Weaponising of the material aside, it greatly increases the risk of the babies dying in a slightly rough landing.
