What I mean with Mysterio being the next big bad, might be a little exageration. But Mysterio is a very important villain for the future of the MCU. As in his last words: "People need something to believe, and right now they'll believe anything". This is a dark tease of what is coming and what has come in the MCU. The population of the world was punched in the gut several times with the Avengers actions, by discovering that life can suddenly vanish with alien invasions, cities falling down the sky and snaps that turn half of the world into dust. Mysterio is a response to how all of these impossible stuff change people's perceptions of the world. People are craving for heroes and if heroes is what they want, then if a random warrior of an alternate dimension with a fishbowl on his head comes along, they'll accept it easily. Now the world's biggest threat,,, is belief.