I don't see how this is possible. They've made Tom Holland's Spider-Man cannon in the MCU. He's an MCU character. Anyone he interacts with is in the MCU, period. There's no grey area. People are going to associate him with the MCU after Homecoming and Civil War. If he shows up in a Venom movie, people are going to think Venom is in the same universe as the Avengers. (From a generic moviegoers perspective.)
Marvel will not make that mistake, he's in or he's out. And if he's in, they're going to have a big impact on what he does.