Falcon

It might be hard for some of the flagship MCU characters to show up in a Spider-Man film, especially with big events like Ragnarok on the horizon, so why not have some of the second-tier heroes, like Anthony Mackie's Falcon, show up instead? Falcon's personality would mesh incredibly well with Peter Parker's, and the visuals of the wall-crawler swinging around the city with Falcon flying by his side are a superhero fan's dream.