I think it would be smart for Sony not to sell Spider-Man outright to Marvel, but to sell the license to him for a one-off Avengers appearance or something. Raise the awareness/cool factor of your series by showing him with the other Marvel heavy weights, and then benefit from the Avengers boost. Financially, that would be best idea for Sony. This way, they can keep making money off Spider-Man films. If they sold him, they'd make a large chunk at one time and then would make no future profit. That being said, if Sony wanted to do that, they need to deliver higher quality.
Like I said, when SM3 came out, everyone wanted to see this with me. I ended up seeing it 5 times because it felt like everyone in the world wanted to see it with their amazing best bud Spider-Fan. This movie, I saw it once and likely won't again. My family/friends don't care about these films like they did the prior ones. My brother, who is in England currently and loves comics as much as me, has yet to see this, and we discuss all these movies. Yet, he hasn't seen it yet. I'm not saying this in order to prove the last series was better. This is just illustrating to me that the enthusiasm for the character himself just isn't currently there. Sony needs to get that enthusiasm they squandered with SM3 onward back.