Am I wrong in remembering that originally he was meant to be one of the bad guys in the last Spider-Man movie and that was originally the tease for setting that up, but then things changed and nothing came of that aspect?
I mean the tease is literally in the last Spider-Man movie (No Way Home), and the scene where a chunk of Venom is left behind happens at the end of the movie. When Eddie/Venom are sent back to their world (as a result of the spell sending the movie villains back, although no one in the movie knows about Eddie/Venom having been brought to the MCU) they leave a chunk of venom behind, which seems to be alive. Since it involves a chunk of the Sony venom symbiote being left in the MCU it can only be teasing a Venom/venom born symbiote in the MCU, presumably in a potential Spider-Man 4