I would really like to see Ben or Kaine on the Avengers primarily because of how Spider-man is used on those teams. Peter is either a jokester or he's the tech guy when Iron man isn't around. So in this scenario you put someone on the team who is just as smart but applies it in a more grounded way like Ben. This can also lead to scenarios where Ben might be able to emphasize with a villain in order to stop them rather than giving a heroic speech or punch them. I'd have Ben get to know characters like you know how you'd have a supervillain prison? Ben is the warden. Ben gets to know the bad guys and help them climb out of the Hell's they live in because he's lived that and been on the receiving end of Kaine. He knows what it's like to not be in your right mind and because he's been on both sides of the fence he's seen how that affects people as much as the people they fight.
Avenger is a term for exacting punishment for a wrong so with all these heavy hitters you have that guy who is going to play "good cop" with you and because he's not Peter who keeps people at arms length, he's going to be with that villain every step of the way.
At least that's my spontaneous pitch if he could be put on a team.