To be honest, the ending bothered me a little. In my opinion, it’s only OK to end on a cliffhanger if the story is tied up enough that you could be satisfied if the next one never came. For example, both Empire Strikes Back and Infinity Wars both end on a cliffhanger (sort of) in that they both end with the protagonist losing; but in Empire Luke and Leia definitively get away and in Infinity Wars the Avengers definitively lose. While both of those endings are the protagonists’ low points, the movies could be complete even with no sequel

Meanwhile, Across the Spider-Verse’s story is still majorly incomplete and if there was no follow up, it would feel like the first half of a movie forever (though an exceptional one). The interesting thing to me, is I think the movie would feel more complete if they cut material out. Since Gwen’s arc (not having a support system and not feeling loved by her sole remaining family member) was tied up at the end and Miles had the climactic escape from Nueva York, I think the movie would have felt more tied up if they saved all the Earth-42 scenes (including Prowler!Miles) for the next movie. That way both protagonists has a more finite end point for the movie. That’s my opinion, anyway.